I know the question already received good answers, but I want just to comment something: try working first with letters and only in the end substitute the numbers. This means that you would do:
$A^2 + A^2 = C^2 \Longrightarrow 2A^2=C^2$
Hence $A=\pm \dfrac{\strut C}{\strut \sqrt{2}}=\pm \dfrac{\strut C\sqrt{2}}{2}$
Doing so you just need to put the value for $C$ and you get $A=\pm 14\sqrt{2}$ which is the answer. I just wanted to comment that because doing things this way you get practice to work with more general equations and on the other side you reduce the number of numerical calculations needed.