OK, I will give this a go!
In smaller triangle EFC:
Angle FCE =tan(EF /FC) =tan(FCE)=5/(12 - 5)=5/7
Angle FCE=arctan(5/7) =~35.54 degrees.
In the large triangle ABC:
Tan of angle ACB =5/7
5/7 =BA / AC
5/7 =BA / 12
BA =12 x 5/7 =8 4/7 units.
BA - DA =BD
8 4/7 - 5 =3 4/7 units.
