Gravitation formula for time?

Question

Given a universal gravitational constant G, how long in seconds would it take two initially stationary spherical objects with masses m1 and m2 kg and radii r1 and r2 m whose centers are d meters apart to connect in an isolated environment?

Answer 1

F = Gm1m2/ r^2
F = ma
a = v/t
v = d/t
a =d/t^2
F = m d/t^2 = Gm1m2/ d^2
m = (m1 + m2)
F = (m1 + m2) d/t^2 = Gm1m2 /d^2
d/m2t^2 + d/m1t^2 = G/d^2
d/m2 + d/m1 = Gt^2/d^2
SQRT{ (1/G) d^2 (d/m2 +d/m1)} = t