To solve the equation, you would "undo" the division by 3. The way you do that is to multiply both sides of the equation by 3.
Once you know the answer is 36, then you can check that by substituting 36 for p. (Some might call this "solution by guessing" if you were to start with this step.)
Applicable choices are
multiplying both sides of the equation by 3
substituting 36 for p to check the solution