If you want the hypotenuse.... √ [ (leg 1)^2 + (leg 2)^2 ]
If you want a leg ..... √ [ ( hypotenuse)^2 - (leg)^2 ]
Ok, I am going ot give you a simple answer, the pythagorean theorem. Which is a2 + b2 = c2
So if you have a normal right triangle, like this one, you'd use the theorem. First you'd do 52 + b2 = 122.
The reason we don't know what b squared is because that is what we're finding. 52 = 25 and 122 = 144 so fill in the equation. 25 + b2 = 144. Now subtract 25 from both sides. Now you get b2 = 119. Now you have to remove the squared in b2. Since you have to remove the power, you have to find the square root of both sides. So in b2 = 119, do the square root to both sides. So now you get your answer as b = 10.908. If you just want the longest side(hypotenuse), you do a2 + b2 = c2. I am going to pick random numbers, let's say a is 5 and b is 6. So, 52 + 62 = c2. 5 squared is 25 and 6 squared is 36. So 25 + 36 = c2. 61 = c2. So now you have to find the square root of both sides. The answer is 7.81.