Gravitational fields

Stuck on this one would appreciate some help, go wild

My attempt and the question are in attachments

It's easier to do it from the point of view that at the zero point the field due to the M is equal to the field due to 4M. So

$\frac{GM}{y^2} = \frac{G.4M}{z^2}$

where z is the distance from this point to the other (4M) mass.

Can you work it out from here?

Is that not what i've done but instead of z I used d-y?

Yes, sorry I should have said it was ok, but you have not done the last step.
I thought it would be easier to see it using z.

Sorry I still don't understand where to go from my last step? i need y/d not y/(d-y)

From the square root of 1/4 you know that y/z (in my example) is 1/2

So you have the distance d divided up into two portions (y and z) such that z is double y.

M|-----y-----|------------z-----------|4M

M|-----1-----|------------2-----------|4M

So what is the ratio of the total length to y?

Really helpful thanks I get it now 1/3