Here's why:

* Adenine (A) pairs with Thymine (T)

* Guanine (G) pairs with Cytosine (C)

So, following the base pairing rules:

* A in AGGTAC pairs with T in TCCATG

* G in AGGTAC pairs with C in TCCATG

* G in AGGTAC pairs with C in TCCATG

* T in AGGTAC pairs with A in TCCATG

* A in AGGTAC pairs with T in TCCATG

* C in AGGTAC pairs with G in TCCATG