Linear Algebra - I'm Stuck ona Question

Hey Guys, I'm kind of embarrassed to ask this question but I've attempted it a few times now and can't afford to waste anymore time on it. I just can't seem to start it properly I think I've over thought it. anyways here is the question verbatim:

Based on her market research, Jill has discovered that the number of glasses of lemonade sold in a day is a function of the sale price (p=p(x), where x is the price, in cents, of a glass of lemonade).

a.) Jill noticed that 75 glasses of lemonade were sold when the price of a glass was set to 80 cents. If Jill assumes that the number of glasses sold is a linear function of the sale price, what should p(x) be?

Well good luck guys. I hope someone can push me in the right direction

regards,