I don't like these algebra problems which pretend to be geometry.
These are supplementary angles adding to 180 degrees; geometry done.
7x - 1 + 3x - 9 = 180
10 x = 190
x = 19
ABC=3x - 9 = 3(19) - 9 = 48
Answer 48°
77(7) = 539, the total points of all her tests
539 - 18 + 81 = correct total points = 602
602/7 = 86 which is the new average
I swear if it’s wrong I’m sorry ok but it’s 3 for me