Fall break homework

I don't understand "the point of diminishing return." The textbook says that to find it, one must seek the point at which the graph is increasing the most, which seems like it would be the middle of the positive portion shaped like a 'J'-- looking somewhat exponential. However, it goes on to say that any f(x) values after this will have the lowest value in relation to x--- Isn't this only true for the f(x) max?