The Leg of a Right Triangle solver, partner to my Hypotenuse of a Right Triangle Solver. I will put a link in the comments to that project if you came to the wrong one. This will find the missing side of the right triangle, NOT THE HYPOTENUSE!
ONLY WORKS WITH RIGHT TRIANGLES. If you want the non-decimal answer, then take the "Missing leg squared" and put a radical in front of it. Don't forget to simplify!