Answer :

You need to work this out using pythagoras' theorem.

Csquared=Asquared+Bsquared

But you are finding out A, not C, so you must rearrange the equation.

Asquared=Csquared-Bsquared
Asquared= 484-256
Asquared=228
A=Square root of 228
A=15 :) 
the correct answer to your questin is 38