Question 1018209
Account for perimeter of the uncut rectangle:
{{{2x+2y=34}}}
{{{highlight(x+y=17)}}} when that equation simplified.


The dimensional measurements for the base surface after the four cut squares removed and flaps folded become  {{{(x-2*2)}}}  and {{{(y-2*2)}}}.


The given diagonal value, through the THREE dimensions follows Pythagorean Theorem formula:  {{{highlight((x-4)^2+(y-4)^2+2^2=7^2)}}}, since the diagonal was given as 7.


The equations outlined in red color form a system to solve.  
The 3-d diagonal equation should be simplified first...
{{{system((x-4)^2+(y-4)^2=45,x+y=17)}}}, and go from there...