I assume you mean to ask how to find the sum of the seven terms?
You have
[tex]a_2=ra_1[/tex]
[tex]a_3=ra_2=r^2a_1[/tex]
[tex]a_4=ra_3=r^3a_1[/tex]
...
[tex]a_7=ra_6=r^6a_1[/tex]
So the sum is
[tex]a_1+ra_1+r^2a_1+\cdots+r^6a_1=a_1(1+r+r^2+\cdots+r^6)=a_1\times\dfrac{1-r^7}{1-r}[/tex]
You know that [tex]a_1=-11[/tex] and [tex]r=-4[/tex], so the sum is equal to
[tex]-4\times\dfrac{1-(-4)^7}{1-(-4)}=-13108[/tex]
