Question

the printed matter on a 14 by 18 centimeter page of a book must covor 140 square centimeters. if all margins are to be the same width, how wide should they be?

Answer 1

Let
x----------------> width of the margin
we know that

[Area printed matter]=140
[Area printed matter]=[(14-2x)*(18-2x)]
then 
[(14-2x)*(18-2x)]=140--------> (252-28x-36x+4x²)=140
(252-64x+4x²)=140----------> 4x²-64x+112=0

using a graph tool---------> I solve the quadratic equation
see the attached figure
x1=2
x2=14----------> is not solution

the answer is 
the margin should be 2 cm