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.