The sum of all angles in a triangle is 180º. Therefore:
[tex]\begin{gathered} 180=\text{ 30 + \lparen3x-10\rparen + x} \\ 180=\text{ 30 +3x -10 + x} \\ 180\text{ = 20 + 4x} \\ 180\text{ - 20 = 4x} \\ 160=\text{ 4x} \\ \\ \frac{160}{4}=\text{ x} \\ \\ 40=\text{ x} \end{gathered}[/tex]
X will be 40º.
