If the sequence of one single strand of dna is c-a-a-g-t-a-g-g-c-t, what is the complementary strand?

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)