Answer:
The common ratio of this geometric sequence is -1/2, so we have
a(7) = 16 × -1/2 = -8, a(8) = -8 × -1/2 = 4.
Dividing by -1/2 is the same as multiplying by -2. Working backwards, we have:
a(5) = -2 × 16 = -32
a(4) = -2 × -32 = 64
a(3) = -2 × 64 = -128
a(2) = -2 × -128 = 256
a(1) = -2 × 256 = -512