Since quadrilateral ABCD is congruent to quadrilateral STUV, and STUV is a rectangle, the lengths of the diagonals of rectangle STUV are equal and can be calculated using the properties of rectangles.
In a rectangle, the diagonals are equal in length. Since you stated that diagonal \( SU = 5 \) cm, it follows that the length of \( TU \) (the other diagonal) is also \( 5 \) cm.
By the Transitive Property of congruence, if \( ABCD \cong STUV \), then the lengths of the corresponding diagonals \( AC \) and \( BD \) must also be equal to the lengths of the diagonals of rectangle \( STUV \).
Thus, we have:
\[ AC = 5 \text{ cm} \] \[ BD = 5 \text{ cm} \]
Therefore, according to the Transitive Property, \( AC \) and \( BD = 5 \text{ cm} \).