In congruent quadrilaterals HWXK and LMDB, side MD is equal in length to side KH. Since KH is given as 12, MD must also be 12.
The question involves finding the length of side MD in a pair of congruent quadrilaterals. When two figures are congruent, their corresponding sides are equal in length.
Since Quadrilateral HWXK is congruent to Quadrilateral LMDB, each corresponding side of one quadrilateral is equal to the corresponding side of the other quadrilateral.
Therefore, if KH = 12 in Quadrilateral HWXK, then its corresponding side MD in Quadrilateral LMDB would also be 12.