I assume you drew the triangle and rectangle just to get a picture?.
The width of the rectangle is x and the height is y.
The area of the rectangle is A=xy.
We know y=-3x+7
So, the area is
Now, by calc or , we can solve for x.
is the width.
Plug this into y to find the height.
Then multiply the two answers to get the area. I bet you get 49/12.