Given the function ƒ(x ) = 3x + 1, evaluate ƒ(a + 1).

F(x) = 3x +1 so than f(a+1) will get easy just substituted the (a+1) in place of x inside equation in this way 

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

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

f(a+1) = 3a +4

hope this will help you 

The answer is