Question

find the value of each of the two coefficient angles is 20°

Answer 1

 the measures are x and y degrees, and they are supplementary, then their sum is 180 degrees. So you can write and solve a system of equations.
x + y = 180
x - y = 20
Now you can use any method to solve. Let's use the elimination method by adding the equations together.
x + x + y - y = 180 + 20
2x = 200
x = 100
Now solve for y.
100 + y = 180
y = 80
So the angles measure 100 and 80 degrees