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:
![x+2x+21=180](https://img.qammunity.org/2023/formulas/mathematics/high-school/61w8hi0e9k4miskthjzg94vwtx9jeu27rl.png)
Combine like terms:
![3x+21=180](https://img.qammunity.org/2023/formulas/mathematics/high-school/k1mio37s9z0xiaodvyw7amxbpny1rcp92g.png)
Subtract 21 from both sides:
![3x=159](https://img.qammunity.org/2023/formulas/mathematics/high-school/s370xv4jx6vabq10qpcduvfgd0nc9h4231.png)
Divide both sides by 3:
![x=53](https://img.qammunity.org/2023/formulas/mathematics/high-school/qhuxair4t7y624xam8z3pj71de0tvbsoau.png)
Therefore, the value of x is 53 degrees.