How do i do this someone explain i need

So.. she washes cars and charges 15 bucks per car...ok... Revenue( sales income), in this case is how much she gets in total income from the car washing.. .well, if she washes, say "a" automobiles, then she makes 15*a, or 15a in income, or Revenue.

now, she pays in advertising 145 bucks... and for cleaning products, she spends 3 bucks for each washed car... ok.. .so if she washes say "a" automobiles, then she spends 3*a or 3a, ok.... so, her costs for the washing cars business is 145 for ads plus 3a, or 145 + 3a is her Cost.

now, Profit is the surplus amount after you remove the cost expenses, and can only happen if the Revenue is higher or greater than the Cost.

Revenue > Cost
  15a      > 145 + 3a

when will she turn a profit?



she will turn a profit after washing 12 cars and 1/12 of a car, maybe just the windshield =).... anyhow..... namely, the revenue then is greater than the cost after that, or one can say, after the 13th car washed. 

2.4+3.2t. you get the answer by adding the ones with variables and ones without