Answer :
Given: It is given that the length of the painting is 24 inches and the width is  11 inches.
To find: Area of the mat
Solution:
The watercolor painting is 24 inches long by 11 inches wide.
So, the area of the painting is:
[tex]\text{Area of painting}=l\times b[/tex]
[tex]\text{Area of painting}=24\times 11[/tex]
[tex]\text{Area of painting}=264 \text{ in}^2[/tex]
The length of painting with mat is = 24 in + 3 in + 3 in = 30 in
The width of painting with mat = 11 in + 3 in + 3 in = 17 in
[tex]\text{Area of painting with mat}=30\times 17[/tex]
[tex]\text{Area of painting with mat}=510 \text{ in}^2[/tex]
Now to calculate the area of mat subtracts the area of painting from the area of the mat.
[tex]\text{Area of mat}=\text{Area of painting with mat}-\text{Area of painting}\\[/tex]
[tex]\text{Area of mat}=510-264[/tex]
[tex]\text{Area of mat}=246 \text{ in}^2[/tex]
Hence, the area of the mat is 246 in².