Correct answer: 6/5 or 1 and 1/5 or 1.2 (pick your favorite form)
This actually technically can't be answered without more information as all we know is that the point (250, 300) exist somewhere on this function describing royalties. However, it is safe to assume that James earns $0 when he is selling 0 books, so the point (0,0) also exists.
Using this information, the formula for slope (rate of change) is rise over run, more mathematically expressed m = (y_2 - y_1) / (x_2 - x_1)
In this case, m = (300 - 0) / (250 - 0) = 300/250 = 6/5 = 1.2