Because it says that Alex wants to spend exactly $30 so you want to have 30 as the total ( = 30 )
and it says 18 plus, you only add 18 one time ( 18 + )
and then it says 0.12 per mile, so you need to do something times 0.12 + 18 and get exactly 30. so you do ( 0.12x ) in this case we're not solving it we're just doing the equation so it's just 0.12x instead of a specific number
