try assigning
Small:
x and y width and length
Large:
w and L width and length
Develop expressions and equations:
xy=wl-80,l=w+5,xy=70
.The two area equations give something to solve.
wl-80=xy=70
wl-80=70
wl=70+80
wl=150
The area for the current large monitor.
Substitute the formula transcribed for L into that area equation.
w(w+5)=150
(w+15)x(w-10)=0
The meaningful value is w=10
l=15.