Since angles that add up to make a line add up to 180 degrees, you know
Angle ACD + angle ACB=180
Since it tells you ACD is 100, substitute that in.
100+ACB=180
ACB=80
Next, the sum of all the interior angles in a triangle is 180.
ACB+A+B=180.
Substitute in the values you know.
80+A+60=180
A=40
Angle a is 40 degrees.
Hope this helps!
