Why the Formulas Work · Medium problem

A right triangle has legs $6$ and $8$. Fay finds the hypotenuse:

Step 1. $c = \sqrt{6^2 + 8^2}$ Step 2. $= \sqrt{36} + \sqrt{64}$ Step 3. $= 6 + 8 = 14$

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