Question 164191
The problem that is presented here is asking you to determine the total cost of the gasoline by plugging in a value for the given variable: number of gallons (g).

When you see a problem in the form:
C(g) = 3.03 * g
The left side of the problem represents the notation for a function.  In this function you are asked to calculate the total cost of 2 gallons of gas and 9 gallons of gas if the cost per gallon is $3.03.
 
In other words calculate for g = 2 and g = 9.
Problem 1: 
C(2) = 3.03 * 2
C(2) = 6.06

Problem 2: 
C(9) = 3.03 * 9
C(9) = 27.27

The number in the parentheses is simply a convenient place to store the value for "g" while you are plugging into the right hand side of th equation.  No need to add or subtract it!