You still do the Pythagorean theorem. Which is a^2 + b^2 = c^2. So for example, say b = 16 and c = 20. You would do:
A^2 + B^2 = C^2
A^2 + 16^2 = 20^2
A^2 + 256 = 400
-256. = -256
A^2 = 144
A^2 = radical 144
Which is a perfect square so it becomes 12.
So the answer is A=12 (B=16,C=20)
Hope this helps :)