Since we're looking for f(x)=0, we're looking for y=0 since y and f(x) can be interchanged. In other words, we are looking for the x-intercept, since y=0 for all x-intercepts. So we substitute 0 in for f(x) and we get:
Now we solve for x
Add 12 to both sides
Divide both sides by 3. This will isolate x.
So if we let x=4 we should get f(x)=0, in other words, f(4)=0. So lets verify this:
Check:
Plug in x=4
works
This verifies our answer
Here you can see the graph of and the x-intercept of (4,0)