Let DF=X, then AD=2X, AF=√3X.

DF=EF=X

Ae = 30-5 = 25m，AE=AF+EF。

∴AE=√3X+X=25

X = 9.2m。

Cd = ef+EB = 9.2+5 = 14.2m。

(the key to solving the problem is that ∠DEF is an isosceles right triangle, DF=EF)

Ask again if you don't understand. I wish you progress in your study and go up a storey still higher! (*^__^*)