Gravitational field strength at point A between two masses

[Lavace]

Please check the attachment.

The question is:
Find the gravitational field strength at the point A in terms of G.

So I went about assuming we use g = GM/r^2, for both masses, and then subtract them as we're meant to find the overall using vectorial addition.

But, the answer in the back of the book is G/4. I can't seem to get this whatsover.

Could someone run through the calculations please?
Thanks a bunch!

 

[Doc Al]

Show what you've done so far. Explain your diagram. What does X mean? What is R?

 

[physixguru]

Both mass and the distance cannot be represented by 'X'.