What is the formula for Newton's First Law of Motion?

First law: The velocity of a body remains constant unless the body is acted upon by an external force.

So, if F is the net force acting on the body and a is the body's acceleration (the rate of change of its velocity) then the first law says that:

"If F = 0 then a = 0."

This in fact follows from Newton's second law, which states that F = ma (where m is the body's mass).