A formula is a machine with labelled sockets. $v = \dfrac{d}{t}$ (read aloud: v equals d over t) takes a distance and a time; $a = \dfrac{\Delta v}{t}$ (a equals delta-v over t) takes a change in speed and a time. Say them out loud once — a formula you can SAY is a formula you own. The fastest way to pick the right machine is to ask what the story GIVES you — not what it asks for.

Steady motion, no speeding up? That is $v = \dfrac{d}{t}$ territory. Something gaining or losing speed? That is $\Delta v$, and $\Delta v$ means acceleration. Nothing has a mass in this chapter, so if you ever feel tempted by $F = ma$ here, the problem is telling you to reread it.