answer: x = 19
assume that the line under the triangle is a straight line, and the angle of any straight line = 180°,
[tex](5x+1)+(8+4x)=180[/tex] collect like-terms
[tex]9x+9=180\\[/tex] subtract
[tex]9x=171[/tex] divide
[tex]x=\frac{171}{9}\\[/tex]
[tex]x=19[/tex]
to make sure of the answer, plug [tex]x[/tex] in the equation:
[tex](5(19)+1 )+(8+4(19)=180\\95+1+8+76=180\\180=180[/tex]
