If an object is not accelerating what can you determine about the sum of all the forces on the object?

Ah hah !

Newton's second law of motion:
 
                     Net force = (mass) x (acceleration)

If the object's acceleration is zero, then the right side
of that formula is zero.

If the right side is zero, then the left side is also zero.

That means the net force on the object ... the sum of
all forces that may be acting on it ... is zero.  The whole
bunch of um have the same effect on the object as if
there were no forces at all.

The car would move to the right with 150 n of force. the 150 n of force from friction and air resistance slows the car from moving to the right with 300 n of force.