[tex]m\angle BEC=70^{\circ^{}^{}}[/tex]
1) As we can see there is a linear pair with the angles ∠BEC and ∠CEF these angles are supplementary. So we can write out the following equation:
[tex]\begin{gathered} 3x-5+4x+10=180 \\ 7x+5=180 \\ 7x=180-5 \\ 7x=175 \\ \frac{7x}{7}=\frac{175}{7} \\ x=25 \end{gathered}[/tex]
2) So now that we know the quantity of x. Let's plug x=25 into the angle ∠BEC formula:
[tex]m\angle BEC=3(25)-5=70[/tex]
