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?