Answer:

Step-by-step explanation:
The function is given.

To find : 
The input for the function f(x) is (a + 1).
Replace x with (a + 1).

Expand brackets.

Simplifying.

If you graph it, it looks linear.
Step-by-step explanation:
step 1. slope = rise/run
step 2. the rise is the height between any 2 points
step 3. the run is the distance left or right between the same 2 points
step 4. if the line goes up "left to right" the slope is positive, down it is negative
step 5. slope = + 6/2 = 3.