Seemingly easy question... too easy.

Suppose the Earth is a plane described by the equation $\displaystyle x + y + z = 18$. Now suppose that at some arbitrary point in time there will be an explosion at the point $\displaystyle P = (1,1,1)$ in space. "City A" will be the first place on Earth that is destroyed by the explosion. With this information, calculate the coordinates of "City A".

It would seem that the first place on Earth destroyed would be directly underneath the explosion, in which case you have (1,1,0). But this is far too easy. Anyone have any other ideas?