I need help understanding this how to solve this word problem.

Question

We're supposed to use graphing to solve this problem, but I can't figure out how to get it into y=mx+b form. 

 

Marys car is 10 years old and has a problem. The repair man indicates that it will cost her $1200 to repair her
car. She can purchase a different, more efficient car for $4500. Her present car averages about $2000 per year
for gas while the new car would average about $1500 per year. Find the number of years for when the total
cost of repair would equal the total cost of replacement.

Answer 1

If x = the number of years, y = the cost.

 

If her current car costs $2000 per year, that's 2000x. Two years will cost 2000(2) = $4000 in gas.

 

Add the $1200 in repairs and we have y = 2000x + 1200. The y-intercept is 1200, how much she will spend before driving it and using any gas. After 1 year, the cost will be $3200, year 2, $5200, etc.

 

I bet you can figure the other!