hello :
the general term is : an = ap×q^(n-p)...... q : common ratio
n=5 and p=2 : a5 = a2×q^(5-2)
768 = -12q^3
q^3 = 768/(-12)
q^3 = - 64
but : 64 = 4^3
q^3 =(-4)^3.... -4^3 =(- 4)^3
q= - 4
calculate the first term : a1
a2 = a1 q
a1 =a2/q
a1 = -12/-4
a1 = 3
the sum of the first five terms is : S= a1 × (1- q^n)/(1- q)
a1 = 3 q =-4 n =5
S = 3(1-(-4)^5)/(1-(-4)).....calculate
