check the picture below.
the left and right are just two rectangles of 5x * 2x
the front and back are just two rectangles of 3x * 5x
the top and bottom are also two rectangles of 3x * 2x
since the area of a rectangle is just width * length then,
[tex]\bf \stackrel{left~right}{2(5x\cdot 2x)}~~+~~\stackrel{front~back}{2(3x\cdot 5x)}~~+~~\stackrel{top~bottom}{2(3x\cdot 2x)}
\\\\\\
2(10x^2)+2(15x^2)+2(6x^2)\implies 20x^2+30x^2+12x^2\implies 62x^2[/tex]
