"We are going to resolve that problem."
Translation:Nosotros vamos a resolver ese problema.
As any Spanish speaking girl will inevitably tell you whenever it comes up, problema is ALWAYS masculine. As to why it cannot be "eso", you can't use "eso" in front of a substantive. So you would never say eso carro, eso libro, etc, always ese. An example for eso: Eso es bueno.