Our variable force is given by 4a^(-2). We shall integrate the force and find the work done at the respective points. Intregal (4a^(-2)) = 4 integral (a^(-2))
So -4(a^(-2+1)) = -4a^(-1) = -4/a
At point 1 and 12 we have -4/a
-4/12 - (-4/1) = -
This gives us -1/3 + 4 = 3.67