Prices:
Fuel x: $2 per gallon
Fuel y: $3 per gallon
"You have at most $18 to spend on fuel" => This means that the sum of the partial costs (for x and y) should be less than or equal to $18. If we have x gallons of fuel x, and y gallons of fuel y, then:
Cost for x: 2x
Cost for y: 3y
The linear inequality is:

Since x and y are the numbers of gallons of fuel x and y, respectively, they must be positive (or zero). Then, the system of linear inequalities is:
