Given the Right Triangle shown in the picture (a Right Triangle has an angle that measures 90 degrees) you can find the length of the missing side "x" by using this Trigonometric Function:
[tex]\tan \beta=\frac{opposite}{adjacent}[/tex]
In this case, you can identify that:
[tex]\begin{gathered} \beta=72\degree \\ opposite=18 \\ adjacent=x \end{gathered}[/tex]
Then, you can substitute values and solve for "x":
[tex]\begin{gathered} \tan (72\degree)=\frac{18}{x} \\ \\ x\cdot\tan (72\degree)=18 \end{gathered}[/tex][tex]\begin{gathered} x=\frac{18}{\tan (72\degree)} \\ \\ x\approx5.8 \end{gathered}[/tex]
Hence, the answer is:
[tex]x\approx5.8[/tex]
