I don’t know the answer to your question, but both of these functions reference an a variable that isn’t passed as an argument. You typically want to avoid referencing globals in Julia (unless a is a const).
Also, forgive me, but this reads a bit like a homework question. There are surely people here that can help (have you seen FFTW.jl? Or JuliaHub), but I think being a bit more specific with what you’re asking might help. Eg, what is your input (using a MWE)? What have you tried? What is the error you’re seeing?
This has some good tips for asking questions.