Math Help - Physics question-how to find maximum accel. given coefficient of static friction...

I'm not sure if I can post here, but I thought I saw a physics question here a time or two.
Here's my question:

A flatbed truck is carrying a 20.0-kg crate along a level road. The coefficient of static friction between the crate and the bed is 0.400. What is the maximum acceleration that the truck can have if the crate is to stay in place?


Is the equation to solve for maximum acceleration F s,max= usN * us?



I was not taught how to do problems like this...not 100% sure what to plug in for usN...I gave it a shot:
The surface is flat, so I guess the normal force=20.0 kg, so I then mulitplied by .40 to get 8 m/s^2.........ahhhh. I don't know what I'm doing!

In simple Physics exercises, if you see mass and acceleration in the same Problem, think at once of the formula F = ma. Force = mass*acceleration.

What is the force that will move the crate for any acceleration?
Why, F = (20 kg)*a = 20a newtons, of course?

What will prevent the crate from moving?
Friction.
Force due to friction = (0.400)(20kg *(9.8newtons /1 kg) = 78.4 newtons.

So if F = 20a is to overcome the 78.4 N friction force, to move the crate, then
20a > 78.4
a > 78.4 /20
a > 3.92 m/sec/sec

Therefore, the flatbed can accelerate to a maximum of 3.92 m/sec/sec before the crate starts moving. -------------answer.