hello :
the nth term of a geometric sequence is :
Un = Up ×r^(n-p) . r is the common ratio
for : p=5 and n= 2
U5 = U2 ×r^3
16 = -2 r^3
r^3 = -8
but : -8 = (-2)^3
so : r = -2
Un = U2 × r^(n-2)
Un = -2 ×(-2)^(n-2)= (-2)^(n-2+1)
the nth term of a geometric sequenceis : Un = (-2)^(n-1)