Tlg bantu jawab yaaahh

CE / CB = DE / AB
4 / (4+x) = 12 / 21
4 × 21 = 12 (4 + x)
84 = 48 + 12 x
12x = 84-48
12x = 36
x = 36/12= 3

jadi BC adalah (4+3)= 7 cm