Hello there. To solve this question, let's call the average hourly operating costs as C and the average hourly receipts as R.
We get that 90R - 10C = 800, since this is the amount the bakery has spent on equipments and it is said he will soon get back this invested money.
Dividing both sides by 10, we get 9R-C = 80
The graph will be a line. We want to find then C = R, so we can get:
-R + 9R = 80
8R = 80
R = 10
So, we have a cost of $100 and receipt of $900 in order to get back that money.