3.a). Use the triangle that you drew for problem #1 to calculate
the mass of this stone.
You know that its weight (Fg) is 3.0 N, and that its weight is (mass)x(g) .
Solve 3.a exactly the same way you solved #2.
They're the same problem, only with a different number for weight.
(And while you're looking back at problem #2,
please add the UNITS to your answer.)
3.b). The net force is the sum of all the individual forces
on the stone, AND their directions.
There are two forces acting on the stone: 12N up and 3N down.
Can you add those ? And get the net force of 9N up ?
3.c). Now it's time for " F = m A ".
Net Force = (mass) x (acceleration) .
Divide each side by (mass), and then you have
Acceleration = (net force) / (mass) .
You've already solved parts 'a' and 'b', so you KNOW the net force
and the mass. All you hafta do to solve part 'c' is pluggum in.