Question 13344
Since you know that the angles are supplementary, you know that they add together to get 180 degrees.  You can add the equations together to equal 180, so you get the equation {{{12x + 1 + x + 10 = 180}}}.  Combine like terms to get {{{13x + 11 = 180}}}.  Then solve the equation to get:

{{{13x = 169}}}
{{{x = 13}}}