I have discovered the solution for the differential equation. I have:
DSolve[{y'[x] == 3 + 1/x}, y, x]
and got
{{y -> Function[{x}, 3 x + C[1] + Log[x]]}}
I'd like to make y a function of z, for example. I just would like to be able to use the function another time, evaluating y[2] for...