Answer:
45
Explanation:
We are seen with two lines crossing each other and the angle in which a line has is 180 degrees. Also, the angles labeled 3x and x are supplementary angles, which mean they add up to 180 degrees. Using this information, we can set up an equation.
![x+3x=180](https://img.qammunity.org/2020/formulas/mathematics/middle-school/kfw6br9snihz7g77uxpjo2628k9qsjjgvv.png)
![4x=180](https://img.qammunity.org/2020/formulas/mathematics/middle-school/h5i6olff05v2i0c0fvosxcb3t9qtgy3250.png)
![x=45](https://img.qammunity.org/2020/formulas/mathematics/middle-school/o4ct7sya36f7y7xhmjmyk9lpxfpbk253ye.png)