Hey there,
Let's say 'L'=length, 'W'=width, and 'H'=height.
The surface area of a rectangular prism is
A = (2LW) + (2WH) + (2LH) = 2 (LW + WH + LH)
If you shrink the width and height, then the new width is W/7
and the new height is H/7.
The new surface area is
A = (2 L W/7) + (2 W/7 H/7) + (2 L H/7) .
That's A = 2 (LW/7 + WH/49 + LH/7) .
or you could write it as A = 2/7 (LW + WH/7 + LH)
There are many more ways to massage the formula and write it
in different ways, but I think writing it just that way is OK.