Question

Carlos pushes a 3 kg box with a force of 9 newtons. The force of friction on the box is 3 newtons in the opposite direction. What is the acceleration of the box?

Answer 1

friction = 3 N 
pushing force= 9 N
as both are opposite in direction so their net force will be = 9-3= 6N
as we know that
F = ma 
re arranging the equation
a = F/m
by putting values 
a =  6/3 N/kg
a= 2 m/sec²

Answer 2

The 'net' force acting on the box is (9 - 3) = 6 newtons
in the direction that Carlos is pushing.

Force = (mass) x (acceleration)

6 = (3) x (acceleration)

Divide each side by 3 :

2 m/s² = acceleration