Given:
Two masses m and 4m kept at a distance r
Let gravitetional potential is zero at a point p
x2Gm−(r−x)2G(4m)=0
x1=(r−x)2
r−x=2x
3x=r
x=r/3
Potential at point p
V=−(2r/3)4Gmm−(r/3)Gmm
V=−Gm(r6+r3)=−9rGm