1N is the same as 1kgm/s2. F= ma. For m and a, enter the numbers in the appropriate units.
For m, solve F = ma: m=F/a.
Plug in some numbers for F and a, with proper units, starting with m = F/a. Substitute what I said the N is equivalent to for the N. Cancel as much as you can until kg = kg. It's effective!
