How Do You Calculate Acceleration with Friction Forces Involved?

[StaticShock]

I am not getting forces at all. I 'm not sure of what to do. Here's the question:

A box is resting on a flat surface. The box has a mass of 20KG and a force of 200N is pushing the box to the right. The force of friction acting on the box is 100 N to the left. What is the acceleration of the box?

I know that the sum of the forces=Mass X Acceleration. But I am not sure what numbers to put in.

 

[Chi Meson]

Consider the forces that act along the line of motion (left-right forces that is , the box is not moving up or down is it?). Call forces to the right "positive. Call forces to the left "negative." What is the sum of all the left-right forces? (Hint: it involves a little mathematical trick called "subtraction.")