Please jawab pake cara

AlasΔkecil / alasΔbesar = sisimiringΔkecil /sisimiringΔbesar
Maka--> x / (x+6) = 12 / 21 (dikali silang)
(21)(x) = (12)(x+6)
21x = 12x + 72
21x-12x = 72
9x = 72
X = 72 /9
X = 8 cm