Findmax returns a tuple, as you can check:
julia> r = rand(10);
julia> findmax(r)
(0.7466769692672559, 7)
And .- is element-wise subtraction, and [2] takes the second element:
julia> findmax(r) .- (2,3)
(-1.253323030732744, 4)
julia> findmax(r)[2]
7
ps. It would be easier to read if you could quote your code.