Hi!
We can see the river is a straight line.
This means that the angles that fall upon it are supplementary angles, and will add up to 180 degrees.
Therefore:
[tex]x+2x+21=180[/tex]
Combine like terms:
[tex]3x+21=180[/tex]
Subtract 21 from both sides:
[tex]3x=159[/tex]
Divide both sides by 3:
[tex]x=53[/tex]
Therefore, the value of x is 53 degrees.
