Question

A DNA strand has the sequence ATCGGC. Which is the complementary strand of DNA

Answer 1

A pairs with T, C pairs with G.

So in your original strand, everywhere you see an A in the new strand you will have a T, and so on.

For example, the first base is an A. So the first base in the new strand is a T. The second base in the old strand is a T, so the second base in the new strand will be an A. Do the rest yourself.