The only thing you need to know in order to solve this task is that plank length (which is force x), should equal the increase in potential energy, so what we have now : (mass)* g * (height).
It has to look like that:
F * 3.0 = 150 x 9.81 x 1.20
Then solve for F, the result should be in newtones = 588N
Do hope it makes sense.
