For f(x) =3x + 2, find f(x) when x=4 and when x= -1

Find f(x) when x = 4
Subtitute the x with 4 in the function
f(x) = 3x + 2
f(4) = 3(4) + 2
f(4) = 12 + 2
f(4) = 14
The value of f(x) when x = 4 is 14

Find f(x) when x = -1
Subtitute the x with -1 in the function
f(x) = 3x + 2
f(-1) = 3(-1) + 2
f(-1) = -3 + 2
f(-1) = 1
The value of f(x) when x = -1 is 1

Line ab is y = -7/5x + 56/5i don't understand  where point d comes into this. 

Ithink