f(a + 1) - f(a) = ??

Question

f(x) = 3x + 5

so how would i do:
f(a+1) - f(a) = 3(?) + 5

Answer 1

You know that f(x) = 3x+5. So, just plug a+1 and a into the formula:

f(a+1) - f(a)
(3(a+1) + 5) - (3a+5)
3a + 8 - 3a - 5
3

So f(a+1)-f(a)=3