In $7y - 21 = 63$, what operation is being done to $7y$? What is the opposite operation you can use on both sides to get rid of $-21$?

After you undo the $-21$ on the left-hand side, you will have an equation of the form $7y = \text{(some number)}$. Compute that number and look for it on the right side of one of the answer choices.

Type 63 + 21 into Desmos and note the result. This is the number that should appear on the right side after you add 21 to both sides of $7y - 21 = 63$ to isolate $7y$.

Rewrite the simplified equation in your head as $7y =$ (that result from Desmos), then select the answer choice whose right-hand side equals that same number.

Two equations have the same solution if they are true for the same value of $y$. That means any value of $y$ that makes the original equation $7y - 21 = 63$ true must also make the new equation true, and vice versa.

In the equation $7y - 21 = 63$, the term $21$ is being subtracted from $7y$.

To isolate $7y$, do the opposite operation, which is adding 21 to both sides:

On the left side, $-21 + 21$ cancels out, leaving just $7y$.

Now simplify the right side:

So the equation equivalent to the original one is $7y = 84$, which matches choice C.