Question 221212
Let the width of the border around
the picture  = {{{x}}}
I can express the width of the mat as
{{{16 + x}}} and the length as
{{{25 + x}}}
{{{(16 + x)*(25 + x) = 792}}}
{{{400 + 25x + 16x + x^2 = 792}}}
{{{x^2 + 41x - 392 = 0}}}
Solve using quadratic formula
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}}
{{{x = (-41 +- sqrt( 41^2-4*1*(-392) ))/(2*1) }}}
{{{x = (-41 +- sqrt( 1681 + 1568 ))/2 }}} 
{{{x = (-41 +- sqrt( 3249 ))/2 }}}
{{{x = (-41 +- 57)/2 }}}
{{{x = 16/2}}}
{{{x = 8}}} (reject the negative solution)
The border will be 8 in wide
check answer:
{{{(16 + x)*(25 + x) = 792}}}
{{{(16 + 8)*(25 + 8) = 792}}}
{{{24*33 = 792}}}
{{{792 = 792}}} 
OK