AACGTGC
There are four bases in DNA. A(adenine), C(cytosine), G(guanine), and T(thymine). For each base, there's one complement. C will only bond with G and A will only bond with T. So let's write down the sequence with each base on it's own line and then the complementary base afterwards.
T <-> A
T <-> A
G <-> C
C <-> G
A <-> T
C <-> G
G <-> C
So the complementary base sequence to TTGCACG is AACGTGC.
