If the total area of a picture measuring 10 inches by 12 inches plus a matting of uniform width surrounding the picture is 224 square inches, the width of the matting is a: 2 inches b: 2 4/11 inches c: 3 inches d: 4 inches

Let
x-----------> uniform width surrounding the picture

we know that
(10+2x)*(12+2x)=224-----> 120+20x+24x+4x²=224
4x²+44x+120-224=0
4x²+44x-104=0

using a graph tool-----> to resolve the second order equation
see  the attached figure

the solution is
x=2 in

the answer is
2 inches

What's the denominator

That they are numbers and related to each other.