Actually managed to figure it out just about
Assume that f(z) = u(x,y) + i v(x,y).
We know that u(x, y) = ϕ(v(x, y)).
By Cauchy-Riemann, we have
∂u/∂x = ∂v/∂y ==> ϕ'(v(x,y)) * ∂v/∂x = ∂v/∂y, and
∂u/∂y = -∂v/∂x ==> ϕ'(v(x,y)) * ∂v/∂y = -∂v/∂x.
Substituting the first equation into...