Question

How to find the the angle measurement ?

Answer 1

That is a 180 degree angle (the straight line) and one angle is 78, so you subtract 78 from 180
So the angle would be 102 degrees

Answer 2

When finding an angle, it's always important to know a rule.
The rule in this case is 'Angles on a straight line sum to 180'

So in this case there is a 78 degree angle. 
180 - 78 = 102 degrees for the remaining angle.