How to determine what sign (+/-) to use?

[nothanksjeff]

Homework Statement

General question, but specific question here: "What is the maximum acceleration a car can undergo if the coefficient of static friction between the tires and ground is 0.80?

Homework Equations

I used Newton's 2nd laws and solved

The Attempt at a Solution

I guess my real issue is knowing which side of the free body diagram to place a force. I placed the static friction force on the negative side of the x-axis, and ended up with -F_s (static friction) = ma , which obviously leads to a negative acceleration. The answer is a positive acceleration, but I'm just not sure how to tell what side it should be on. Does it even make a difference?

 

[haruspex]

A car accelerates from rest. Which way does the force of friction act on the tires? (Hint: what other horizontal forces are present?)