First, we should understand the spring constant. Different springs may have different spring constants. A spring constant tells us that how much force is needed to bring a 1-Meter stretch in a particular spring. Here the force applied is 75 N and the stretch is 0.3 m. Now we divide 75 by 0.3 and we get 250 N/m. So 250 N force is required here to bring a 1 m stretch in this specific spring. So, the answer is 250 N/m. :).
