Please help me on this question

DE = 13.4 cm  (to 1 decimal place)

Step-by-step explanation:

Given: ABCD is a square

BC = AC = 12 cm    (opposite sides of a square are congruent)

E is midpoint of BC   (given)

BE = EC  = 12/2 = 6 cm

CD = AB = 12 cm   (opposite sides of a square are congruent)

angle ECD is a right angle (interior angles of a square are 90 deg.)

Consider right triangle ECD

DE = sqrt(EC^2+CD^2)     pythagorean theorem

= sqrt(6^2+12^2)

= sqrt ( 36+144 )

= sqrt (180)

= 2 sqrt(45)

= 13.416 (to three dec. places)