Point b coincides with point d, so the crease EF is the middle vertical line of BD.
So being educated,
Angle DBC= angle BDE=45 degrees, angle bed =90 degrees.
That is, DE is the height of the trapezoid.
The trapezoid is an isosceles trapezoid with AD = 2 and BC = 8.
So EC=(BC-AD)/2=3.
BE=BC-EC=5
The tangent of the angle CDE =EC:DE=3:5=3/5.