So you know that angle I is verticle angles to the 60 degrees angle
And you known that the verticloe angle to 20 is an important part
Then you use the sum of interior angles formula
which is all the angles in the triangle should add up to 180
20 + 60 + m< HEI = 180
80 + m< HEI = 180
-80 -80
m< HEI = 100
I believe the measure of the angle is 100, I hope it makes sense too!