KI + Pb(NO3)2 →
Pbl2 + KNO3
How many molecules of Potassium nitrate (KNO3) are produced when 6.57 moles of Lead (II) nitrate
(Pb(NO3)2) react?

OK, so the downward force on the rock is mg (force of g) - 2.8N (force in opposite direction). since mg (9.6 kg*9.8 m/s^2)= 94.08 N, then mg-2.8= 91.28 N. That is your net force on the rock (or total force). And, since f=ma, a= f/m.
so, a= 91.28 N / 9.6 kg = 9.5 m/s^2.(final answer)