Question

AB - BC = 12 cm y AB/BC = 5/2

Answer 1

1) AB -BC = 12
2) AB/BC = 5/2 despejamos AB
AB = 5BC/2 remplazo este valor en 1
5BC/2-BC = 12
(5BC-2BC)/2 =12
3BC = 12*2
3BC= 24
BC= 24/3
BC= 8
en uno
AB -BC = 12
AB -8 = 12
AB = 12+8
AB = 20

COMPROBACION

AB -BC = 12
20-8 = 12
12  = 12