m<ABC and m<ABD are supplementary angles and as you know sum of supplementary angles = 180°
so m<ABC + m<ABD = 180°
3x - 9 + 7x - 1 = 180
Combine like terms
10x - 10 = 180
Add 10 to both sides
10x = 190
Divide both sides by 10
x = 19
Plug in x = 19 into 3x - 9 to find m<ABC
m<ABC = 3x - 9 = 3(19) - 9 = 57 - 9 = 48
Answer
m<ABC = 48°