ACGTCGTAGCTG
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Since you are working with DNA, not RNA, no uracil is present. Therefore, all A's code for T's and all T's code for A's. All C's code for G's and all G's code for C's.
ACGTCGTAGCTG ---> TGCAGCATCGAC
ACGTCGTAGCTG ---> TGCAGCATCGAC