Here's how to determine the complementary mRNA sequence:

1. Remember the base pairing rules:

* Adenine (A) pairs with Uracil (U) in RNA

* Thymine (T) pairs with Adenine (A) in DNA

* Guanine (G) pairs with Cytosine (C)

2. Apply the rules to the DNA sequence:

* TTA AGG CC (DNA sequence)

* AAU UCC GG (mRNA sequence)

Therefore, the complementary mRNA sequence would be AAU UCC GG.