Here's why:

* Base pairing: In DNA, Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This is called complementary base pairing.

* Antiparallel orientation: The two strands of DNA run in opposite directions (antiparallel). This means that if one strand is 5' to 3', the other is 3' to 5'.

Therefore, to find the opposite strand sequence, we simply replace each base with its complement:

* A becomes T

* G becomes C

* C becomes G

* T becomes A

This gives us: GTCGAGT