How to get demand equation

Hello,

it's probably a silly question for you, but I'm stuck.

It's simple. There's given a table of prices and quantities and I need to get a demand equation from it. I know the answer, but I would like to know the 'HOW-TO'.

Q ‖ 1 ‖ 2 ‖ 3 ‖ 4 ‖ 5 ‖ 6 ‖ 7 ‖

P ‖ 35‖30‖25 ‖20‖15‖10‖ 5 ‖

Answer: Q = 80 - 2P.

What's on my mind: I understand we can calculate the slope, which is -1/5 and then we should do something I don't know ( well I've read that we can put the slope to p=mx+b and solve it, but it didn't give me the right answer). Also it can be found using matrix algebra, which I'm curious about.

Thanks in advance.