How do I use triangle sum properties to find unknown values?

The first step in order to find the unknown value of the triangle sum is to know that the sum of the measures of the interior angles of a triangle it has to equal  180. In order to know if this is equivalent to 180 you have to  use the facts that are given to you on the problem.When you are trying to find the unknown number you have to use the Triangle Angle Sum Theorem to solve for it which it is 

 after you have put the equation you need to substitude into the equation and if it is correct it has to equal 180.