1)
[tex]\bf \textit{surface area of a cube}\\\\
s=6x^2\quad
\begin{cases}
x=\textit{length of a side}\\
---------\\
s=150
\end{cases}\implies 150=6x^2\implies \cfrac{150}{6}=x^2
\\\\\\
25=x^2\implies \sqrt{25}=x\implies \boxed{5=x}[/tex]
3)
[tex]\bf \textit{volume of a square pyramid}\\\\
V=\cfrac{b^2h}{3}\quad
\begin{cases}
b=\textit{base's side}\\
h=height\\
-------\\
h=3\\
V=25
\end{cases}\implies 25=\cfrac{b^2\cdot 3}{3}\implies 25=b^2\cdot \cfrac{3}{3}
\\\\\\
25=b^2\cdot 1\implies 25=b^2\implies \sqrt{25}=b\implies \boxed{5=b}[/tex]
