These angles are opposite from each other, meaning they are the same.
Note: I'm pretty sure they are called opposite angles, but it's been a long time.
If they are the same, you would need to use an equal sign.
So the equation is x+40 = 3x
Now solve the equation. To remove the x from the left side, you subtract x.
(x+40)-x = (3x)-x
Note: Ignore the parentheses, they are used so you can clearly see the problem more (That doesn't make sense LOL).
Now we have 40 = 2x
Now we have to isolate the x, by removing the 2. Since 2 is multiplied to x, we have to divide it from both sides.
40÷2 = 2x÷2
You get 20 = x
Therefore, the value of x equals 20°
