The area of a rectangle is given by the formula:

Where b is the length of the base and h is the height of the rectangle.
The area of a triangle is given by the formula:

Where b is the length of the base of the triangle and h is its height.
The resulting shape on the right is a rectangle, whose base is 2 and its height is 4, then the area of this part of the shape is:

The area of the triangle resulting on the left side is:

Since the area of both shapes is 8, Clare is right.