Angle DBA and angle ABC are supplementary angles.
The supplementary angles add up to 180°.
Therefore we have the equation:
m∠DBA + m∠ABC = 180°
(7x - 1) + (3x - 9) = 180
7x - 1 + 3x - 9 = 180 combine like terms
(7x + 3x) + (-1 - 9) = 180
10x - 10 = 180 add 10 to both sides
10x = 190 divide both sides by 10
x = 19
m∠ABC = (3x - 9)°
Substitute:
m∠ABC = (3 · 19 - 9)° = (57 - 9)° = 68°
Answer: m∠ABC = 68°
