Perhatikan gambar berikut! Panjang AB adalah

FB = x

36/36+27 = x/x+18
36(x+18) = x(36+27)
36x+648 = 63x
648 = 63x - 36x
648 = 27x
648/27 = x
24 = x

FB = x

AB = FB + AF
AB = 24 + 18
AB = 42 cm

( B )