ABCDA, B, C, D is a square and \overline{AC}=30\text{ cm}.

Question

AC
=30 cm.

Answer 1

Ok, that's good to know.
What about it?

Meanwhile "ABCDA, B, C, D is a square" is a statement
which makes no sense.

Answer 2

that's just that annoying repetition of copy/paste or dictation
ABCD (A, B, C, D) is a square ...