The requested equation is
V(t) = 180 - 25t gallons,
where V(t) is the remained volume of water, t is the time in hours.
The formula says that the initial volume of water is 180 gallons (at t= 0)
and that 25 gallons is drained each hour.
To answer another question, make calculations ON YOUR OWN by substituting the value t= 4.5 hours to the formula.