Answer:
18 gallons
Explanation:
Let x represent a full tank, then
x + 9 = x
Multiply through by 10 to clear the fractions
2x + 90 = 7x ( subtract 2x from both sides )
90 = 5x ( divide both sides by 5 )
18 = x
The tank holds 18 gallons
9.5m questions
12.2m answers
x + 9 = 
x