How is it possible for the Ukraine to win the war, considering that they receive help from the americans and european.
First, there has to be a set of goals Ukraine agrees would constitute winning the war. That could mean pushing Russia out entirely, keeping as much of their land as possible, taking some of Russia's land, or economically bankrupting Russia.

The various sanctions around the world have already worked towards bankrupting Russia, however, Russia is an exporter of fertilizer, food, and oil, so it's impossible to remove the food and energy bases of their economy with sanctions alone. They also export aluminum, copper, and gas turbines, so removing their economic ability to create war machines may be impossible with only economic sanctions as well.

Taking some of Russia's land from before the war could be risky, as it would look so bad on Putin and other Russian leaders that they could respond with more force. They already view Ukraine as a loss from the former Soviet Union, and additional losses could be viewed as threatening the existence of a nuclear capable nation, especially if the areas to be taken have strategic value or are near Moscow.

Pushing Russia out entirely may be possible given enough tanks and manpower, but Russia has over three times the population of Ukraine. So each Ukrainian unit would need to kill over three Russian units before dying, or two thirds of Russia's citizens would have to flee to other countries before being drafted. This may be possible with Russia's corrupt military, its sending tanks into urban areas where anyone with a anti tank rocket launcher can hide, and their initial lack of personnel due to calling it a 'special military operation' at first and not sending reserves. However, as time goes on, these corrupt and ineffective parts of Russia's military may literally be killed off.

Keeping as much of their land as possible would require determining the point where they are still most effective against Russia, what exactly Russia would be happy to achieve with their war, and the desire to negotiate. However, President Zelensky has told news reporters that he wasn't interested in more peace talks with Russia, and his previous 10-point peace plan, which included justice for Russian war crimes and restoring Ukraine's state borders with Russia, was rejected by Russia, so the desire to negotiate may be nonexistent.

The most viable win then may be pushing Russia out of Ukraine entirely, which would require continued support from the west in terms of war machines like tanks and drones, as well as potentially troops, which may come from Private Military Contractors to hide the west's full involvement. This may become harder and more costly as the war drags on though. A better strategy may be to find or provide ways for Russians that don't want to fight and die in a war to escape Russia.
Is there a method/formula/theorem to calculate the probability of either side winning perhaps bayes theorem or something similar?
No, the probability of either side winning depends on many non-quantifiable factors. For example, US military aid to Ukraine is dependent on continued US congressional support. The tenns of billions of dollars in aid will certainly be a controversial topic in the upcoming 2024 US elections.