since AY bisects it, both of the equations they give you equal the same thing, so you can set them equal to each other.
8x-4 = 5x+20
subtract 5x from both sides
3x-4=20
add 4 to both sides
3x=24
divide both sides by 3
x=8
now plug in 8 to one of the equations
5(8) +20
40+20
60
you then have to multiply that by two to get the measurement of the large angle.
60*2=120
the final answer is 120°