We have to consider that the DNA nucleotides, Adenine (A), Cytosine (C), Guanine (G), and Thymine (T) are classified into pyrimidines, and purines, which refers to the number of carbon rings they have in their structure, being one for pyrimidines (Cytosine, and Thymine), and two for purines (Adenine, and Guanine). Pyrimidines and purines are complementary to each other, but not between themselves, in other words, we always have a pyrimidine matched with a purine and not a pyrimidine with another pyrimidine, nor a purine with another purine. We can also say that between these molecular structures, there's a chemical affinity in the links they form between their hydrogen bonds, which only allows a Thymine to match with two hydrogen bonds with an Adenine, and only allows a Guanine to match with three hydrogen bonds with a Guanine.
We can see this graphically in the following picture:
Considering this, if we have the strand of the first question, which is the following:
T A C G T A A C G C A T C G G
Their complementary strand would be the following:
A T G C A T T G C G T A G C C
And it would look like this:
T A C G T A A C G C A T C G G
A T G C A T T G C G T A G C C
