Can someone explain this to me please thank you !!

The figure can be drawn as,
From the triangle ABC,
[tex]\begin{gathered} \sin 70=\frac{AB}{AC} \\ \sin 70=\frac{h}{400ft} \\ h=400\sin 70ft \\ h=375.87ft \\ \approx376ft \end{gathered}[/tex]Thus, the required value of height is 376 ft.