Answer with Explanation:
The forces are unbalanced because the engine force (bigger force) and the friction force (smaller force) are not equal. Their difference in force is 40 N. This makes the truck move forward or in the direction of the bigger force. Its movement is being slowed down by the friction force.
Both of the forces are acting in opposite directions, but they are not equal in magnitude. Such occurrence can make an object either slow down or speed up. It could also change its direction. This is in accordance to "Newton's First Law of Motion."