The key words here are "constant speed". By this, we mean that all the forces on the object (the sailing boat here) are balanced - so they must be equal in magnitude.
Here, we know that the boat has a forward force of 300 N. Because the forces are balanced, then the backward force must also have a force with magnitude 300 N.
However, note that because the backward force is in the opposite direction as the forward force, we must put a negative sign on the 300. So our answer is: -300 N.
