Answer:
DE = 19.5
Explanation:
the diagonals of a rectangle are congruent and bisect each other, then
EC = AE , that is
7x - 5 = 3x + 9 ( subtract 3x from both sides )
4x - 5 = 9 ( add 5 to both sides )
4x = 14 ( divide both sides by 4 )
x = 3.5
Then
AE = 3x + 9 = 3(3.5) + 9 = 10.5 + 9 = 19.5
Thus
DE = AE = 19.5