The box is moving at constant speed. Therefore, acceleration is 0. Since acceleration is 0, resultant force can be derived using the formula,
F=ma
=>F=mx0
=>F=0 N
If the resultant force is 0, that means the pulling force is equal to the frictional force because they cancel each other out because since they are acting in the opposite directions. Therefore, frictional force is 30 N.