Us to prove that BE = (AB theta)/((alpha - theta)).

From the diagram, we observe the following:

• The square symbol at A indicates that $\angle EAB = 90^\circ$. This means $\triangle ABE$ is a right-angled triangle at A.
• The angle $\theta$ is shown between the line segment BE and a horizontal line extending from B (let's call it BC). So, $\angle EBC = \theta$.
• The angle $\alpha$ is shown as $\angle AEB = \alpha$.