a cube has all equal sides, so every face in the cube is just a square, if the area of one of those square is 25 cm², that simple means,
[tex]\bf \textit{area of a square}\\\\
A=s^2~~
\begin{cases}
s=length~of\\
\qquad a~side\\
------\\
A=25
\end{cases}\implies 25=s^2\implies \sqrt{25}=s\implies 5=s[/tex]
now, look at the picture below, a cube is really just 6 squares stacked up to each other at the edges, in this case, all 6 squares are 5x5.
so, simply get the area of all of them, and sum them up, and that's the surface area of the cube.
(5*5)+(5*5)+(5*5)+(5*5)+(5*5)+(5*5).
