They are supplementary so the total of the two angles, by definition must be 180 degrees. Note that you should understand that any number of angles can but supplementary as long as they all add up to 180 degrees.
Solve for x<span>: </span> Remove the Parenthesis. Simplify. Add -4 to both sides. Simplify. Add 8x to both sides. Simplify. Divide both sides by 12. Simplify.