Answer:
15 months
Explanation:
y represents the total cost in the equation. The words per month tells you that is where the x will go. 75 doesn't change you have to pay it no matter what so it will not have the x with it.
y = 15x + 75
To solve, plugin 300 for y, then solve for x.
300 = 15x + 75
300 - 75 = 15x + 75 - 75
225 = 15x
225/15 = 15x/15
x = 15 months