Let's say the length of the small box is x. We assume it's a cube, so all the dimensions are equal.
Now, the volume of the cube equals the product of its dimensions:
V = length . length . length = length³
So, the volume of the large box is:
[tex]V_L=(3x)^3[/tex]As we know this volume is 243, we have:
[tex]\begin{gathered} 243=(3x)^3 \\ \sqrt[3]{243}=3x \\ x\text{ = }\frac{\sqrt[3]{243}}{3} \end{gathered}[/tex]Now you use the value of x to find the volume of the smaller box:
[tex]V_s=x^3=\frac{\sqrt[3]{243}^3}{3^3}=\frac{243}{3^3^{}}=\frac{243}{27}=9[/tex]