$F = ma$ — F equals m a, net force in newtons, mass in kilograms, acceleration in $\mathrm{m/s^2}$ — is one fact wearing three outfits: solve it for $F$, for $m$, or for $a$, and it answers three different exam questions. The ladder is the one you know: pick the formula, rearrange it for what you want, put the numbers in their sockets, and only then compute.

And run the free check first: $\mathrm{kg \cdot m/s^2}$ is exactly a newton, so $F = ma$ survives the units test — that is no accident, it is where the newton COMES from. Push the units through your rearrangement every time: if they refuse to cancel into the answer's units, the rearrangement is wrong, no appeal. Units that do work out never prove you right — the check is a one-way street.