Colton and his dad bought a gallon of paint that cost $13. they also bought 2 brushes that cost $9 each. what was the total cost, not including tax, of the brushes and the paint they bought?

The first thing we are going to do for this case is define variables:
 x: gallons of paint
 y: brushes
 We then have the following expression:
 f (x, y) = 13x + 9y
 For x = 1 and y = 2 we have:
 f (1, 2) = 13 (1) + 9 (2)
 f (1, 2) = 13 + 18
 f (1, 2) = 31
 
 The total cost, not including tax, of the brushes and the paint they bought is:
 $ 31