Similar problems, basically we must find similarities, and the solutions are as follows:

Because: angle BCD=90 degrees;

So: angle BCF+ angle DCF=90 degrees;

Because: angle BFC=90 degrees;

So: angle BCF+ angle CBF=90 degrees;

So: angle CBF= angle DCF

To sum up: triangular BCF is similar to triangular DCE；; ;

Similarly, the triangular AED is similar to the triangular BFA；; ;

Similar to each other: cf/de = BF/CE = > cf/5 = BF/7 = > 5bf = 7cf—(1)

BF/AE = AF/DE = =〉BF/3 =(3+EF)/5 = =〉5BF = 9+3EF = =〉5BF = 9+3 *(7-CF)————(2)

(1), (2) Meanwhile,

It is obtained that CF=3, BF=2 1/5=4.2.