Basically in each of the two equations you have a linear relationship and a constant. The constant can be looked upon as an initial cost, and the linear relationship can be looked upon as a cost that increases as more of a good is purchased. So, for example say you have to spend 650 for a car mechanic and then 10 per hour, versus spending 400 plus 60 per hour. By setting them equal to each other, you are determining after how many hours it would be just as expensive to do one as the other.