You can solve right angle triangles using the pythagorean theorem, in which a^2 + b^2 = c^2, where c is the largest side.
Rearrange this to c^2 - b^2 = a^2
giving you 5^2 - 3^2 = A^2
in which 16 = a^2
and to solve for just a, square root both sides to give you a final answer of a = 4
