Please simplify the first step

Question

Two bodies of masses m and 4m are placed at a distance r. The gravitational potential at a point on the line joining them where the gravitational field is zero is

(a) zero 

(b) − (4(Gm)/r)

(c) − (6(Gm)/r)

(d) − (9(Gm)/r)

Answer 1

Here is the simplification. Simply you have to cross multiply the terms.

Let gravitational field be zero at P as shown in figure.