Two particles, each of mass m, are initially at rest very far apart.

Obtain an expression for their relative speed of approach at any instant as a function of their separation distance d if the only interaction is their gravitational attraction to each other.
Express your answer in terms of some or all of the variables m, d, and gravitational constant G.

I got sqrt(Gm/d) but it says I have an "incorrect numerical multiplier", which I think means something went wrong algebraically?