Both are supplementary angles so their sum will be 180°
[tex]\\ \sf\longmapsto -2x+54=142[/tex]
[tex]\\ \sf\longmapsto -2x=142-54[/tex]
[tex]\\ \sf\longmapsto -2x=88[/tex]
[tex]\\ \sf\longmapsto x=\dfrac{-88}{2}[/tex]
[tex]\\ \sf\longmapsto x=-44[/tex]
