Question 1002544
Basically this will use  {{{Overall-Painting=Frame}}}.


Setup some variables and try to do any computations later instead of sooner.
{{{system(w=29,L=32,A=378,u=unknownFrameWidth)}}}
A is for area of the painting, NOT including frame.


Overall Area, wL
Painting's Area, (w-2u)(L-2u)
Frame Area, assigned to variable A
-
Referring to the basic idea at the top,
{{{highlight_green(wL-(w-2u)(L-2u)=A)}}}


Begin simplifying that equation.
{{{wL-(wL-2Lu-2wu+4u^2)-A=0}}}
{{{wL-wL+2Lu+2wu-4u^2-A=0}}}
{{{2Lu+2wu-4u^2-A=0}}}
{{{-4u^2+2Lu+2wu-A=0}}}
{{{4u^2-2Lu-2wu+A=0}}}
Finish simplying
{{{highlight(4u^2-(2L+2w)u+A=0)}}}


Solve for u.  You can substitute the given values whenever you feel the need.  Your choice either to solve for u completely in symbols, or substitute the given values now and finish that way.