check the picture below.
so... notice "y", is also a vertical angle with the other side.
now, there are a total of 360° total in a circumference... so.... if the left-angle is 68, and the right-angle is also 68, because they're vertical angles, then the remainder is 360 - 68 - 68 which is 224.
now, the remainder has to be split evenly between the two remaing angles, "y" and her twin, thus each one takes half of 224, and thus "y" is half of 224.