1/D=1/z+1/s=(s+z)/sz so D=sz/(s+z) and sD+zD=sz.
As this appears to be a calculus problem, if D=dz/ds, then sD+zD=d(sz)/ds. Therefore d(sz)/ds=sz. Let x=sz, then dx/ds=x. We can write this as dx/x=ds and integrate each side wrt s: ln(x)=s+k where k is the constant of integration, and x=e^(s+k). Replace x with sz and we get sz=e^(s+k), so z=e^(s+k)/s. This can also be written z=Ke^s/s where K=e^k.