if A and B must be parallel, then:
[tex]\angle2+\angle4=180[/tex]
where:
[tex]\begin{gathered} \angle2=2x+10 \\ \angle4=4x+80 \end{gathered}[/tex]
so:
[tex]\begin{gathered} 2x+10+4x+80=180 \\ 6x+90=180 \\ solve_{\text{ }}for_{\text{ }}x\colon \\ 6x=180-90 \\ 6x=90 \\ x=\frac{90}{6} \\ x=15 \end{gathered}[/tex]
