We have to solve for the common ratio first...
The rule for a geometric sequences is:
a(n)=ar^(n-1) and we are given two terms so we can say:
-16/1024=(ar^3)/(ar^0)
-16/1024=r^3
r=(-1/64)^(1/3)
r=-1/4 so
a(n)=1024(-1/4)^(n-1)
a(6)=1024(-1/4)^5
a(6)=-1