j ai un dm de math et j aurai besoin d'aide car j ai rien compris

1) (Bf)//(AE) thales

CB/CA = CF/CE = BF/AE
x/x+5 = 3/5

b) 5x = 3(x+5)
  5x = 3x+15

c) 5x = 3x+15
5x-3x = 15
2x = 15
x =7,5

d) 7,5m