To solve the equation \(-3(y + 6) = 21\), the first step is to isolate the term containing \(y\) by dividing both sides by \(-3\). This gives us:
\[ y + 6 = -7 \]
The second step, after dividing both sides by \(-3\), is to subtract \(6\) from both sides to solve for \(y\).
Therefore, the correct answer for the second step is:
Subtract 6 from both sides.