When you have different forces working or acting on an object, the forces can either work with each other or work against each other. When they are working against each other, as you have suggested with this example, if the forces are equal to each other, no motion will be produced with the block. We would say that the forces are balanced. When the forces are not equal, you subtract the force which is going left from the force going to the right. You also have two forces going to the right, you would simply add those together to get the cumulative effect. So, 100 N + 20 N = 120 N of force pulling to the right. 120 N - 35 N = 85 N, so the block will have 85 newtons of force pulling it to the right. When there are unbalanced forces, such as you have here, the result is motion in a specific direction, usually towards the larger force.