At $7.50 / hour, you would take home pre-tax earnings of $11,700. You likely wouldn't pay a federal income tax on that (though you may state and local income tax). So we'll just call it the $11,700.
Assuming you have NO living expenses (lol). It would take ($14,000 * 4) / $11,700 = 4.8 years to save it up. That's if you bank every single penny you make. That is a ridiculous assumption, and realistically you'd probably spend most of that meager wage just surviving.
I would add that you would be eligible for a federal pell grant to help cover the cost. That would offset perhaps $6,000 / year.
Let's make further assumptions, let's say you get the pell grant at $6,000 per year and you bank 25% of your income. The pell grant lowers the cost to $8000 / year ($32,000 for 4 years). Saving 1/4 of your income amounts to $2,925 / year. Let's say you invest that into a bank account that returns 1% / year compounded daily.
Using the financial calculator here
http://www.financeformulas.net/Number-of-Periods-of-Annuity-from-Future-Value.html
It would take you roughly 10 years to save up the money, assuming no tuition increase.
Realistically, at that hourly rate, you'll have to bury yourself in debt to go to school.