Respuesta :

the formula is:  (AB +BC) * BC = EC*DC
 now fill in the numbers that are given:

(AB +6)*6 = 12*5

(AB+6)*6 = 60

6AB +36 = 60

6AB = 24

AB = 24/6

AB = 4

AC = AB +BC
AC = 4 + 6

AC = 10