Ineed i've been trying but couldn't get it

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!

1.17+3.99a. you combine the terms with a to create 3.99 and the 1.17 stays because it has no like terms or a variable