Raymond was asked to solve for the length of the hypotenuse in a right triangle with legs that have side lengths of 4 and 5. His work is shown below. He made a mistake when solving. Explain the mistake and then solve the problem correctly.

Raphael's Solution:
a ^ 2 + b ^ 2= c^2 (to the power of 2)
4^2 + 5^2=c^2
16 + 25= c^2
41= c