Number from 1 to 9 to create 2 coordinates with a slope of 8. You may only use a number once?

Slope is rise over run. Run is horizontal distance, left to right. Run is always positive because we always go left to right. Rise is the vertical change in that same distance. A negative rise means it drops.

The equation of a line is y = mx where m is the slope. That line passes through the origin. If you want it to pass through some point (a, b) you subtract the coordinates like this: y - b = m(x - a). That is the point-slope form, and you can rewrite it in other forms if it is convenient. You may do this with any point on the line. They all reduce to the same equation.

You are given y=8x + b. The b doesn't matter, so you can set it to any number. Since your domain is limited, you have to set x=1, making y=8. Now your coordinates are (1, 0) and (0, 8) with b=0. That doesn't work. You can't set b=1 because you already used that digit. You can't set b=2 because that would make y=10. No solution exists.