Calculating Roller Coaster Speed at Top of Loop-the-Loop

[sracks]

Homework Statement

The normal force equals the magnitude of the gravitational force as a roller coaster car crosses the top of a 37 m diameter loop-the-loop.
What is the car's speed at the top?

Homework Equations

n-mg=0
I don't know what equation to use

The Attempt at a Solution

I honestly don't know what to do...

 

[Shooting Star]

You have to do something. What's the force called which acts radially inward on a mass going in a circle? And what's the magnitude?