First, ABCD is a square < D = 90 (given)
In square ABCD AD and DC are adjacent sides, and AC is diagonal of square
then..
AC^2 = AD^2 + DC^2

since ABCD is a square, AD= DC (all sides of square are equal)

---> (18.79)^2 =17^2 + 17^2
---> 353.06 = 289+289
---> 353.06a 578

so it contradicts that the diagonal of the square is equal to the root of the square with two sides.