We have $8.50 an hour gross minus $2.15 an hour tax and bennys. Then $3.75 daily expense. In one week our net take-away y after x hours is
y = x(8.50 - 2.15) - 5(3.75)
That's the answer, or we can simplify it to
Answer: y = 6.35 x - 18.75
Answer: The slope, 6.35, is the net dollars per hour before weekly expenses.
Answer: The y intercept, -18.75, is the weekly expense, negative because it's an expense.
To make $120 in a week, we solve for x where y=120
120 = 6.35x - 18.75
138.75 = 6.35 x
x = 138.75 / 6.35 = 21.8503937007874
That wasn't the nice round number I was expecting/hoping for. I don't see a mistake so let's go with
Answer: 21.85 hours to make $120