check the picture below.
the lateral area, namely the area of the sides, as you see in the picture, is really just the area of 6 rectangles.
front and back, two rectangles of 5x25,
left and right, two rectangles of 2x25.
simply get the area of each and sum them up, that's the lateral area of the prism.
[tex]\bf \stackrel{\textit{front and back}}{2(5\cdot 25)}~~~~+~~~~\stackrel{\textit{left and right}}{2(2\cdot 25)}[/tex]
