So I think the proof is straight forward, but looking for verifications:
$$g(z)=v-iu=-i[iv-i^2u]=-i[iv-(-1)u]=-i[u+iv]=-if(z)$$
So $g$ is a multiple of $f$ and thus $g$ would satisfy the Cauchy-Riemann equations as $f$ did, and so $g$ is also analytic.
Is this enough?