Final answer:
The correct next step in the derivation of the quadratic formula by completing the square is step 3: Take the square root of both sides of the equation.
Step-by-step explanation:
The correct next step in the derivation of the quadratic formula by completing the square is step 3: Take the square root of both sides of the equation. Completing the square involves manipulating the equation to write it as a perfect square, then taking the square root to isolate the variable. By taking the square root, we can solve for the unknown value.