How do you find three ordered pairs for - x+2y=-8

For a linear equation like this there will always be infinitely many solutions if you let x and y be any real number. So you can just pick any x and find the corresponding y. Take for exmaple x=1 then we get

-1+2y=8

2y=9

y=9/2=4.5 so an ordered pair that satifies your equation is (x, y) = (1,9/2).

Now you can go on and find more.