Wave Equation Word Problem

At time $\displaystyle t=0$, two explosions occur in 3-space. They are contained within the spheres of radius 1 at the points (4,0,0) and (0,9,0). If you are located at hte point (0,5,0), what is the largest set of times $\displaystyle t>0$ for which you will not hear sounds from either explosion?

OK, so this is a 3-D wave problem, I know that, and I could probably solve it pretty easily. However, I'm having trouble with the setup. Can someone point me im the right direction as far as the setup is concerned?