Why the Formulas Work · Medium problem

A right triangle has legs $3$ and $4$. Fay finds the hypotenuse:

Step 1. $c = \sqrt{3^2 + 4^2}$ Step 2. $= \sqrt{9} + \sqrt{16}$ Step 3. $= 3 + 4 = 7$

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