Surely integrating f(x) to get v(x) is wrong, since f(x) = ma and m=1 then surely we get a = 72/x^3 - 12/x^2. In this case we have acceleration in terms of displacement, so the identity dv/dt = (dx/dt)(dv/dx) is the most useful form for the accelleration. This leads to:
v(dv/dx) = 72(x^-3) - 12(x^-2)
=> (v)dv = ( 72(x^-3) - 12(x^-2) )dx
Integrating this will give an equality involving v^2 - then apply a sqaure root to the other side to find v(x).
At least, I think that's what you should be getting at...
Since all the energy is converted to kinetic energy we can find that E = 1/2(m)(v^2) - 1/2(m)(u^2) which simplifies to E = 1/2(v^2 - u^2).
Now, finally let "n" represent the equation found for the velocity of the particle in terms of x, now we can find that:
v = dx/dt = n . This leads to dt = (1/n)dx . Now once again integrate to get t = int (1/n)dx .
Finally use this integral to find the area between x=4 and x=12 as a definite integral.