Question 965121
x and y, photograph dimensions.
a, the area of just the frame, alone


{{{y=12+x}}}, assuming y is length.
Let u be the uniform width of the frame.


Total area frame plus photograph,{{{(x+2u)(y+2u)}}}
area of just the photograph,{{{xy}}}
area of just the frame,{{{(x+2u)(y+2u)-xy}}}


According to description and assigning,
{{{(x+2u)(y+2u)-xy=a}}}

{{{xy+2yu+2xu+4u^2-xy-a=0}}}
{{{2yu+2xu+4x^2-a=0}}}

and through substituting according to description,
{{{2(12+x)u+2xu+4x^2-a=0}}}
{{{24u+2ux+2ux+4x^2-a=0}}}
{{{4x^2+4ux+24u-a=0}}}
{{{highlight_green(4x^2+4ux+(24u-a)=0)}}}


Now would be a good step to substitute the given values, compute constant and coefficients and then finish solving for x.
Use as described:
x, unknown photograph width
u=5
a=620
y=12+x