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.