Question 462023
Assuming the width of the frame is the same all the way round:

(20-x)(12-x) =  128

240-12x-20x+x^2 = 128

x^2 -32x+240 = 128
x^2-32x+112 = 0
x-16)^2 - 144 = 0

x = sqrt 144 + 16

x = (-12) + 16 (can't be +12 as the dimensions are only 20cm by 12cm)

x= 4