If I understand the question, these two (f(x) and h(x)) are not inverses.
10 is a bit tricky. An inverse can invert into itself: This is called an involution. But if they are differently labeled, I'm not sure. I don't think that this is exactly what is men by an involution. Something like y = x would be. But you have 2 different inverses. I'm not sure that this qualifies.
You're going to want to take the total, which is $353.00 and subtract the initial cost, which is $84.00, the number now should be $269. Using that total divide it by $68.00, there you should find your answer of about 4 hours. The precise answer is 3.99588235. But you would round that to 4 hours.