Gotcha!
Since there is no variables listed, I'll solve it both ways. :-)
f(-18)=2/3(-18)-4
Multiply the fraction by negative 18!
f=-12-4
answer: f=-16
If it's this way then it goes like this:
f(-18)=2/3-4(-18)
f=2/3+72
f=218/3 or f=72.66666..... Rounded: 72.7