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