What you have written makes no sense at all. You ask for the total surface area of a box but say nothing about how "x" and "y" are related to the box. Perhaps x and y are two of the dimensions (length and height, say) but what is the third dimension (width)? Also, you say, "8x+ 4y= 12 also y= 3- 2x" but those are, in fact, equations of same line in the xy-plane.

I would like to help but I really have no idea what the problem reallyis.