Answer: x=17
Explanation:
1.Identify relationship
Since both the angles are on the inside of both parallel lines and on the same side of the transversal you know that you have a same side interior relationship that makes them supplementary
2. set up equation
because they are supplementary you know that they are equal to 180 which gives the equation: 3x+6+123=180
3. solve the equation
3x+6+123=180
3x+129=180 | combine like terms
3x=51 | subtract 123 from both sides
x=17 | divide by 3 on both sides