Lorentz invariance of wave eqn.

Hello! Hopefully somebody could give me a push from behind on this one :)

Homework Statement

Show that the classical wave equation is lorentz invariant.

The Attempt at a Solution

I tried to exchange all derivatives by the chain rule:

$$(c^2 \frac{d^2 }{dt^2} + \frac{d^2 }{dx^2} + \frac{d^2 }{dy^2} + \frac{d^2 }{dz^2}) \phi = 0 ; \quad \frac{d}{dx} \rightarrow \frac{d}{dx}\frac{dx}{dx'}$$
And the same for the time derivative and use lorentz transformation. But somewhere it goes wrong..