All values of angles in a triangle is equal to a total of 180 degrees.
We know that one angle is 72 degrees so we subtract that from 180 to get 108
180 - 72 = 108
We have two missing angles, both equal to y, so we divide the difference of 108 by 2 to get 54 degrees.
108/2 = 54 degrees
72 + 54 + 54 = 180
180 = 180
y = 54 degrees