tolong kasih tau bagaimana caranya

jawabannya sudah disitu ya:) sorry kalau ada yg salah..

a.
COB + BOA = 90°
x + (3x + 10)° = 90°
4x + 10° = 90°
4x = 90° - 10°
4x = 80°
x = 80°/4
x = 20°

b. besar sudut AOB
AOB = 3x + 10 = 3(20°) + 10° = 60° + 10° = 70°