A slope is a surface of which one end or side is at a higher level than another both a rising or falling surface.
And to find the rise over run use the slope to find a second point on the line. You will start with the y-intercept and rise up or down and then run left or right depending on the sign of the slope.
Set up a proportion:
1/7 L of water - 1/9 of the aquarium
x L of water - 1 aquarium (the whole aquarium)
1 2/7 liters of water are needed to fill the aquarium.
