Answer:
21x^2 + 7x
Explanation:
The formula for the area of a rectangle is L x W. The length in this case is 3x+1 and the width is x. Multiply those together and you get 3x^2 + x.
The formula for the area of a triangle is 1/2BH. When you plug in the numbers, the area of the triangle is 24x^2 + 6x.
Now, you want to subtract the area of the triangle from the area of the rectangle. When you do this, you get your answer: 21x^2 + 7x.