Why the Formulas Work · Medium problem

A right triangle has legs $8$ and $15$. Fay finds the hypotenuse:

Step 1. $c = \sqrt{8^2 + 15^2}$ Step 2. $= \sqrt{64} + \sqrt{225}$ Step 3. $= 8 + 15 = 23$

Which step contains the first error? Answer with the step number.