f(x) = 5x + 250
As given in the problem, you need to make a function of the form
f(x) = mx + b.
So first, determine the value of m by taking the difference in y over the difference in x for two points on the desired line. You know that if 0 books are read, Gregory will be donating $250. So one point is (0,250). And if 1 book is read, Gregory will be donating $255. So another point is (1,255). So the slope is the difference in y (255-250 = 5) dividing by the difference in x (1 - 0 = 1), so 5/1 = 5, therefore the function now looks like
f(x) = 5x + b
Now we need to determine b is. For that, just substitute any known pair of x and y values and solve for b. Since we already used (0,250) above, we'll use them again. So substitute into the equation.
f(x) = 5x + b
250 = 5 * 0 + b
250 = 0 + b
250 = b
So the completed function is
f(x) = 5x + 250
