if one strand of DNA has the nitrogenous base sequence ATGCTTGA the sequence in the complementry strand will be TAGGTAGT TACGTACT TACGAACT TCCGAACT

The base pairs on one strand of the DNA bind with the base pairs of the other strand specifically. A always pairs with T with two hydrogen bonds and G always pairs with C with three hydrogen bonds. So if one strand is ATGCTTGA then the complementary strand will be TACGAACT.