Question 1022746
Picture is x by 3x.
Picture's area is  3x^2.


Border has uniform width, surrounding the picture, 2 cms.


Total area is  {{{(x+2*2)(3x+2*2)}}}
{{{(x+4)(3x+4)}}}.


JUST the area of the border is 200 SQUARE CENTIMETERS.
JUST the area of the border is {{{highlight_green((x+4)(3x+4)-3x^2=200)}}}.
Simplify and solve this equation.  You will have x from which to evaluate 3x.


Steps from there:
{{{3x^2+12x+4x+16-3x^2=200}}}
{{{16x+16=200}}}
{{{16x=184}}}
{{{2x=23}}}
{{{highlight(x=23/2)}}}, and you can find 3x from this.