Explanation:
(Assuming that this triangle is isosceles)
If this triangle is isosceles, then x° is going to be equal to its twin angle; 40°.
We can solve for z now.
180 = 40 + 40 + z
180 = 80 + z
Subtract 80 from both sides.
100 = z
z = 100°
Now that we know z = 100 degrees, we can begin to solve the expression (3x -20)
The expression sits on a 180° line and the angle z (100°) shares the line with the expression (3x - 20)°
180 = 100 + (3x - 20)
Subtract 100 from both sides.
80 = 3x - 20
Add 20 to both sides to isolate 3x
100 = 3x
Divide by 3 on both sides.
100/3 = 3x/3
33.33... = x