A thin uniform disk of mass M has outer radius 4r and inner radius 3r. The work required to take a unit mass from point P pn its axis to infinity is?

The gravitational potential at a point P in the gravitational field is defined as the work required to bring a unit mass from infinity to the point P in the field.

Hence the problem is solved if  potential at P is calculated.

Consider a coaxial and concentric ring of radius y and thickness dy 

Find potential at P due ti this element 

Integrate within proper limits. For y the limits being 3r and 4r.

Required W = - (potential)