Question 1084821
The area of the portrait is 50x70 = 3500 square cm
Let's call w the width of the frame.
Its area is 2*(70+2*w)*w+2*50*w = 
{{{140+4*w^2+100*w=3500}}}
 then dividing by 4:
{{{w^2+25*w-840=0}}}

By solving the equation we get w = around 19cm