The water level, w, in feet, of a river after a rainstorm is a function of the time, t, in house, since the storm began.
The water level goes up .4 feet for every .5 hours according to the chart. This also means it goes up .8 feet every hour. Using this as our slope or rate of change, we find that in 0 hours the water level was 18.7-.8= 17.9 feet. This is the original water level. With this information, the equation we get is 0.8t+17.9=w(t).