will be through 4 x4 that's the furthest away so actually going to be able to first look at X 4 and say we're just going to sum over X 4 let's let's forget about X chief for now we'll need to squeeze in there but we'll sum over Explorer so we're left with summation over X 4 and we can actually just do this part there's no Explorer anywhere else the result of that will be something that does involve X 3 because as you some out X for Z's are constants but there's still an extreme here so we need to put on the outside still a summation over X