We have that both pentagons are similar, then, we can write the following proportions:
[tex]\frac{900}{21}=\frac{300}{x}[/tex]
doing cross product, we get the following:
[tex]\begin{gathered} \frac{900}{21}=\frac{300}{x} \\ \Rightarrow900x=300\cdot21=6300 \\ \Rightarrow x=\frac{6300}{900}=7 \\ x=7 \end{gathered}[/tex]
therefore, x = 7
