a) x+58°=180°
x=180°-58°=122°
b) 6x+2x=180°
8x=180°
x=180°/8=45°/2=22°30'
c) 3x+6x+20°=180°
9x=180°-20°=160°
x=160°/9
d) 2x+5x-30°=180°
7x=180°+30°
7x=210°
x=30°
