Question 1000372
Each 'next element' is -2 times the previous element

So lets start with that

{{{(-2)^x}}}  where x>=0 and is an integer. 

If we stop with this we end up with a sequence of 1, -2, 4 , -8 etc
So we are close, but the signs is wrong. So negate each element

{{{- (-2)^x}}}  where x>=0 and is an integer.