Berdasarkan gambar disamping besar sudut B adlah

x + (x + 21) + (x + 15) = 180
x + x + x + 21 + 15 = 180
3x + 36 = 180
3x = 180 -36
3x = 144
x = 144/3
x = 48


B = (x + 15)= (48 + 15) = 63°