Final answer:
The formula a = vf - x/t represents the equation for average acceleration. To solve for t, rearrange the equation by isolating t on one side. The correct quantity in the formula is vf - a.
Step-by-step explanation:
The formula a = vf - x/t represents the equation for average acceleration, where a is the acceleration, vf is the final velocity, x is the displacement, and t is the time interval.
To solve for t in this equation, we first need to rearrange it. Let's isolate t by dividing both sides of the equation by a: t = x/(vf - a).
Therefore, the correct quantity in the formula is vf - a.