The sum of the internal angles of that figure is equal to 720, therefore:
[tex]\begin{gathered} 90+n+n+n+n+90=720 \\ add_{\text{ }}like_{\text{ }}terms\colon \\ 180+4n=720 \\ solve_{\text{ }}for_{\text{ }}n\colon \\ 4n=720-180 \\ 4n=540 \\ n=\frac{540}{4} \\ n=135 \end{gathered}[/tex]
