Force attracting a particle and time required

[LazyMan]

Homework Statement

A particle of mass m is released from rest a distance b from a fixed origin of force that attracts the particle according to the inverse square law:
F(x) = -kx^-2

Show that the time required for the particle to reach the origin is
Pi times (mb³/8k)^1/2

Homework Equations

I have no idea what you need

The Attempt at a Solution

I am sad to say I have no clue how to even star this one. Any help would be nice, just don't give me the answer just yet.

 

[kuruman]

Use energy mechanical energy conservation and solve for dx/dt. After you do that you might see what comes next.