Use the formula to evaluate the series -3+6-12+ formula: sn=a1(1-r^n)/1-r in the formula for a finite series, a1 is the first term, r is the common ratio and n is the number of terms.

-3+6-12+24   <---- notice the terms firstly, they go as

-3 , +6 , -12 , +24 ,....    <---- to get the next term's value, you multiply it by -2

thus, is a geometric sequence, and -2 is the "common difference", and the first term is -3 of course.

so... what's the sum of the first 7 terms?

The density of the cube is 40