Hey there!
The formula for finding the mass is F/A, or Force divided by Acceleration.
Originally, the formula is F = MA (Force is equivalent to Mass multiplied by Acceleration), but I just modified the formula. It's still the same - but we're solving for a different variable.
Since we have our mass and acceleration, let's solve for the mass.
14,000 / 5 = 2,800
This means our mass is 2,800 kg. (Remember your units!)
I hope this helps!