The pythagorean theorem states that a^2 = b^2 + c^2, where a is the side that opposes the right angle.
In this case, you have a and b, so you need c.
That means that c^2 = a^2 - b^2 so c is the square root of (a^2 - b^2) that means c = the square root of (676 - 576) which is 10in.