Answer: 60º
Explanation:
With respect to the line ZB, it makes an angle of 180º if measured using DZ. If the angle outside the triangle measures 140º, subtract from 180º to find the angle inside the triangle.
180º-140º=40º
So, now we have angle c=80º and angle b=40º, we can make an equation to find angles D which we'll call x.
The sum of the angles of a triangle must be equal to 180º.
80+40+x=180
120+x=180
x=180-120
x=60