Sum of a supplementary angle is 180 degrees
3x +5 + 5x +15 = 180
8x + 20 = 180
8x = 180 -20
8x = 160
divide both sides by x
8x/8 = 160/8
x = 20 degrees
3x + 5 = 3(20) + 5
3x +5 = 60 + 5
3x +5 = 65 degrees
5x +15 = 5(20) + 15
5x +5 = 100 +15
5x +5 = 115 degrees
