At time 0, there are 16 gallons of water.
That gives you point (0, 16).
This point is a dot on 16 on the y-axis.
After 1 hour, the water went sown 2 gallon.
At hour 1, there are now 14 gallons left.
This gives you point (1, 14).
To plot it, start at the origin, (0, 0), go 1 unit right on the x-axis, and 14 units up om the y-axis, and place a dot there.
Now connect the two dots and extend the line down to the right until you intersect the x-axis. You should intersect the x-axis at 8.