I don't understand how to do this

Answer:
20 degrees
Step-by-step explanation:
You know that the three angles will add up to 180 degrees, so you can make an equation:
3x (angle 1) + 2x + 20 (angle 2) + 5x - 40 (angle 3) = 180
3x + 2x + 20 + 5x - 40 = 180
Then, solve
combine like terms
10x - 20 = 180
add 20 to both sides
10x = 200
divide both sides by 10
x = 20 degrees
Step-by-step explanation:
3x+5x-40+2x+20=180°{sum of triangles}
10x-20=180
10x=180+20
x=200
10
x=20°
hope it helps.