Having to do with the $1200 per month & $35 for every phone you sell

Shelia made $4000 last month how many phones did she sell?

Since her base amount is 1200, we have 1200+=. Since she gets 35 dollars for every phone, if the number of phones she sells is x, we have 1200+35*x=the total amount of money in a month = 4000. Subtracting 1200 from both sides, we get 2800=35*x and by dividing both sides by 35 we get x=80