Look at the picture.
The angle y° and 71° are Supplementary Angles. Therefore y° + 71° = 180°.
[tex]y^o=180^o-71^o=109^o[/tex]
The sum of measures of these three angles of any triangle is invariably equal to 180°.
Therefore we have the equation:
[tex]x^o+109^o+28^o=180^o\\\\x^o+137^o=180^o\ \ \ |-137^o\\\\x^o=43^o[/tex]
