In the given diagram, identify the parallel lines and angles and use the property of alternate interior angles to solve for x.

The diagram shows two parallel lines, indicated by the arrows. Let's call the top parallel line $L_1$ and the bottom parallel line $L_2$. So, $L_1 \parallel L_2$.
Let the vertices of the triangle be $A$ (the top vertex, where the angle is $4x$), $B$ (the bottom-left vertex), and $C$ (the bottom-right vertex).

Consider the line segment $AB$ as a transversal intersecting the parallel lines $L_1$ and $L_2$.
The angle $3x$ shown at the top-left is formed by $L_