An alien floating around near alpha centauri pushes a small rock with a force of 15 n towards our solar system. if the rock is accelerating at 7 m/s squared, what would the mass of the rock be?

We're almost positive that Newton's laws of motion are true near Alpha Centauri, just as much as they're true here in OUR neighborhood.  So we can use Newton's 2nd law, and assume that

Force = (mass) x (acceleration).

(15 N) = (mass) x (7 m/s²)

We're also confident that elementary algebra works out there too, so we can divide each side of this equation by  (7 m/s²).  Then it says ...

Mass = (15 N) / (7 m/s²)

Mass = (15/7) (N-s²/m)

Mass = 2.143 kg