First you are using the pothagorean theorem which is a^2+b^2=c^2. You have a and b, so you should plug them in to get 4^2+4^2=c^2. Then simplify it some more to 16+16=c^2. Now finish simplifying to c^2=32. Now take the square root of each side and you have your answer. So your answer will be c=√32 or c is about 5.66.