At 4: 00 am, there were 800,000 gallons of water remaining at a reservoir. after 4 hours of irrigation, there were 100,000 gallons of water remaining. write an equation that describes the number of gallons of water as a function of the time the field has been irrigated

Let the presents the time, and the  the gallons of water remaining, so our first point will be (4,800000). We know from the problem that after 4 hours, at 8:00 am, there were 100000 gallons, so our second point will be (8,100000).
To relate those two points we are going to use the point slope formula:

where
is 8
is 4
is 800000
is 100000

So, 



Now that we have our slope, we are going to use the point-slope formula to find our equation:





The equation that describes the number of gallons of water as a function of the time the field has been irrigated is 
where
are the remaining gallons of water in the reservoir after  hours 
is the time the field has been irrigated in hours