So I set up the equation up as X= computers, x+x+12+2(x+12)=220. then you would find the value of x which should be 46. so the 1st week he sold 46 computers, for the second week he sold 58 computers, then 116 the third week.
You can plug them into the x spots, for example, the second week he sold 12 more than he did the last week so it would be x+12= number sold so 58.
To check if you got it correct, plug it back into the equation to see if it equals 220 which it does.
