The question says "If gas in a cylinder is maintained at a constant temperature T". So I don't think T is a variable. Does the answer contain somthing like dT/dV? I don't think so because T isn't a variable.

dP/dV
=d/dV [nRT/(V - nb) - an^{2}/V^{2}]
=d/dV [(nRT/(V - nb)] - d/dV (an^{2}/V^{2})
now take all the constants out to the left hand side of d/dV
=nRT*d/dV [1/(V-nb)] - an^{2}* d/dV (1/V^{2}) .......................(1)

The blue part: [1/(V-nb)] = (V-nb)^{-1}
d/dV [1/(V-nb)] = -1*(V-nb)^{-2} = - 1/(V-nb)^{2}
(the power rule)