To get the length of any one of the sides we need to know the length of the other two. The basic formula will look something like this in your old math text book but I'm going to write it in Max script using our letter representations of the sides.
sc^2 = sa^2 + sb^2
What the above assumes is we already have the length of sa and sb. Lets make some assumptions about our triangle to get started.
sa=10.0 --Length of Opposite side
sb=20.0 --Length of Adjacent side
To calculate sc we will need to remove the ^2 from the left side and add the opposite of that to the right side which will be sqrt (square root).
sc = sqrt (sa^2 + sb^2) --calculates the length of the Hypotenuse.
What we have done above is calculate the length of the hypotenuse. We can use similar functions to get the length of sa or sb if we know the other two. Here they are...
sa = sqrt (sc^2 - sb^2) --Calculates the length of the Opposite.
sb = sqrt (sc^2 - sa^2) --Calculates the length of the Adjacent.