perhatikan gambar berikut .

3x - 10 + 2x = 180°
5x - 10 = 180°
5x = 180°+10°
5x = 190°
x = 190°/5
x = 38°

Jadi, besar sudut ABD = 3x - 10
= 3(38) - 10
= 114 - 10
= 104°