x:
-(rho+k)*(rho+k-1)
+(rho+k+1)*(rho+k)*q
+c*(rho+k+1)*q
-(a+b+1)*(rho+k)-a*b;

y: subst(1-c,rho,x);

factor(coeff(y, q, 0));
factor(coeff(y, q, 1));