Suppose someone opens the valve on a large water tank so that water drains out. choose a starting volume of water in the tank (from 100 to 500 gallons), and choose how much water drains out each day (from 2 to 5 gallons). write the equation that models the relationship between time (x) and water volume (y) in slope-intercept form.

If we are to choose any number from 100 to 500, you may take any other number but for the purpose of showing how things are done here, let me choose 100 gallons. Similarly, we choose 2 gallons to be drain out each day. 

The equation that would best represent the relationship between the given parameters is,
                               y = 100 - 2x

where y is the volume of water after x days.