The parent function of

is

The diagrams below show step by step transformation
As we can see from the graph

, the amplitude of the function is 3, period of

, phase shift of π and midline y=2
Y - y1 = m(x - x1)
slope(m) = 3
(4,7)....x1 = 4 and y1 = 7
now we sub
y - 7 = 3(x - 4) <==
1920/50=38.4 liters of deck stain
3.8 liters in a gallon
So approximately 10.1 gallons or 10.14421 gallons of deck stain :)
22/7 Is what i think it might be