Question 306914
Two Angles are Supplementary if they add up to 180 degrees. 

So, angles (3x + 6)and (2x - 1) add up to 180 degrees. 

{{{(3x + 6)+(2x - 1)=180 }}}
*[invoke explain_simplification "(3x + 6)+(2x - 1)=180" ]