[tex]\text{It's a geometric sequence with:}\\\\a_1=-4\\\\r=8:(-4)=-2[/tex]
[tex]\text{The formula of the sum of a geometric sequence is:}\\\\S_n=\dfrc{a_1(1-r^n)}{1-r}[/tex]
[tex]\text{substitute:}\\\\n=7;\ a_1=-4;\ r=-2\\\\S_7= \dfrac{-4(1-(-2)^7)}{1-(-2)}=\dfrac{-4(1+128)}{1+2}=\dfrac{-4\cdot129}{3}=-172[/tex]
