check the picture below.
the lateral area, namely the area of the sides, is just the area of 4 rectangles.
left and right, 4x3 rectangles,
front and back, 15x3 rectangles,
so the lateral area of the box is just the sum of those areas,
[tex]\bf \stackrel{\textit{left and right}}{(4\cdot 3)}~~~~+~~~~\stackrel{\textit{front and back}}{(15\cdot 3)}[/tex]
