Thank you again Sir!
Just to be sure: ##(1,-4,0),(0,0,1)## is a orthogonal basis of the ##Im(f)## subspace, ##(4,1,0)## is the basis for the one-dimensional nullspace of ##f## and so, by virtue of the rank-nullity theorem, I have an orthogonal basis of the domain of ##f##, i.e. ##\mathbb{R}^3##.