check the picture below.
so, the prism is just 6 rectangles stacked up to each other at the edges.
now, if we get the area of all 6, and sum them up, that's the surface area of the rectangular prism.
[tex]\bf \stackrel{\textit{front and back}}{2(5\cdot 5)}~~+~~\stackrel{\textit{left and right}}{2(5\cdot 12)}~~+~~\stackrel{\textit{top and bottom}}{2(5\cdot 12)}[/tex]
