The tank shown is a cylinder with diameter equal to 4 feet and height equal to 12 feet. the tank holds 1130 gallons of rainwater when full. how many gallons of water are in the tank when the depth of the water is 9 feet?

we know that
volume of the tank=1130 gal

step 1
find the volume of the tank with deep 9 ft
volume of the tank with deep 9 ft=pi*r²*h
r=4/2-----> 2 ft
h=9 ft
volume of the tank with deep 9 ft=pi*2²*9-----> 113.04 ft³

step 2 
convert to gal
1 ft³------------------> 7.48052 gal
113.04 ft³--------> x
x=113.04*7.48052-----> x=845.60 gal

the answer is
845.60 gal

alternative method

if the volume of the tank is 1130 gal for ----------> a height of 12 ft
 X volume-----------------------------------> 9 ft
x=9*1130/12-----> 847.5 gal