For everyone saying work = f * s, this only works when the force is constant, so if the line was horizontal on a force-extension graph. The graph in the exam forms a triangle with the x-axis, so if you're multiplying force and extension at 6cm and getting around 130J then that's just a coincidence and not correct. Work done is technically the integral of force with respect to displacement, i.e. area under a graph, NOT just "force * distance" so you need to use AREA to get the marks