Answer:
$80
Step-by-step explanation:
First I'll ask (cause I'm not an expert at english)(and I'm a child)
Additional fee for maintenance is a thing you pay every month right?
In that case,
Monthly fee per month (m): 11
Additional fee per month (a): 9
Total fee per month (t): [m+a] or [11+9]
Total fee for 4 months (x): (11+9)x4=80
Customers will have to pay $80 after 4 months.
to get the the equation of any straight line, we only need two points off of it, let's use the two points already in the picture.

The answer is -12. Do you need to know how I did it?
<span>I would love to help </span><span>2*12 = 24</span>