m∠A = 42° [isosceles triangle]
m∠B = 180 - 42 - 42 = 96° [in a triangle, the three interior angles always add to 180°]
m∠C = 180 - 96 - (x+12)
m∠C = 84 - x - 12
m∠C = 72 - x
2x+9 + 3x-1 + 72-x = 180
4x + 80 = 180
4x = 180 - 80
4x = 100
x = 100/4
x = 25
m∠C = 72 - x = 72 - 25 = 47°
I hope this helped