Answer:
4.5 or

Explanation:
I selected 2 points on this graph those points being:
(2,9) and (4, 18)
The equation you'd want to use is

so filling in each of the point we'd have this for our equation

when you do the subtraction for both we'd get
Now if you were asked to get a number instead of a fractions you could divide 9 & 2 and get quotient of 4.5
I hope this helps