you just have to replace x with x+2. so it would be
f(x+2)=3(x+2)+4
=3x+6+4 distributive property of equality
=3x+10 combine like terms
Answer is going to be an expression. hope this helps! also if your confused because you think that you would replace ALL of the x's with x+2, and you would keep going on and replacing the x with x+2, and then THAT x with x+2, it helps if you replace the x in x+2 with a different variable, its just there so you know it is a number. so you could do this ad it would be the exact same:
f(x)=3x+4 find f(d+2)
=3(d+2)+4
=3d+6+4
=3d+10
it would be the same answer i know this because i just had this question. :) hope this helps!!