y = 3x - 4
Take the first two points:
(5,11) and (6,14) Note that you could use any of them, but the numbers are smaller for these two.
Slope m = (14-11)/(6-5) = 3/1 = 3
Take either point and use the point-slope formula:
y - 11 = 3(x-5)
Distribute:
y - 11 = 3x - 15
Add 11 to both sides:
y = 3x - 15 + 11
Combine like terms:
Answer: y = 3x - 4
f(17) = 3(17) - 4 = 51-4 = 47
f^{-1}(56) => y=56=3x-4
56=3x-4
56+4 = 3x
60 = 3x
x = 20
Hope this helps.
Jason