OK.

I did add some more to post 3 but I'll show you how to get the y intercept without knowing that equation.

As the gradient is minus 4, that will mean for every 1 you step left in the x direction, the y coordinate will go up by 4.

So step back a few points like this:

(5,0) ... (4,4) ... (3,8) ... (2,12) ... (1,16) ... (0,20)

So the y intercept is (0,20)

Bob