Special Relativity and reference frames

Here's a statement that I'm supposed to solve:

($$\Delta$$x')2 = ($$\Delta$$x)2 - c2($$\Delta$$t)2

And the accompanying text: "Show that (the equation above) is the same for all reference frames in special relativity"

I consider myself somewhat decent with your basic special relativity calculations but I'm having trouble starting this one. I'm not interested in an answer as much as a gentle nudge in the correct direction.

What is this question asking for, mathematically? I don't even know where I should be ending up...