Question 961868
First sentence is confusing.  I will assume you want to say, "the uniform width of the mat surrounding the picture is x inches."


Length of picture,40-2x
Width of picutre,24-2x
Length of total,40
Width of total,24
Perimeter of picture,2(40-2x)+2(24-2x)=64


You can evaluate the length and width of the picture after you have solved for x from the equation  {{{2(40-2x)+2(24-2x)=64}}}.