You can treat x and 2x+1 just as though they are real numbers.
So f(2x+1) = 3(2x+1) - 5 = 6x + 3 - 5 = 6x - 2
In the above, I made use of something called the distributive law which says that if a,b and c are real numbers, then a(b+c) = ab + ac.
