check picture below
use the pythagorean theorem to get the hypotenuse, then get the sine of it
[tex]\bf \textit{pythagorean theorem}
\\\\\\
c^2=a^2+b^2\implies c=\sqrt{a^2+b^2}\qquad
\begin{cases}
c=hypotenuse\\
a=adjacent\\
b=opposite
\end{cases}[/tex]
