Question

Two forces act on a 5-kg object sitting on a frictionaless horizontal surface. One force is 30N in the +x- direction and the other 35N in the -x-direction. What is the acceleration of the object?

Answer 1

Net Force = Mass * Acceleration

Set up a direction convention. Since you already declared +x and -x:

Force (+x) + Force (-x) = Mass * Acceleration
30N - 35N = 5kg * Acceleration
-5N/5kg = Acceleration
-1m/s^2 = Acceleration

Now depending on which direction is negative (left or right?), that is the corresponding direction in which it is accelerating