Since (4y-42) and 2y are opposite angles, they equal to each other which looks like this:
4y-42 = 2y
From there you want to put the y onto one side:
4y-2y-42=0 -----> 2y-42=0
You want to balance the equation out and move the -42 onto the other side so you add it to both sides to get:
2y=42
Then divide both sides by 2 to get:
y= 21
From there you can plug 21 back into the original equation to find each angle.
4(21)-42 = 2(21) ---> 84-42 = 42 ---> 42 = 42
So y = 21 and each angle is 42 degrees