First way of finding area is the usual formula.
Area of a rectangle = Length × Breadth
= 3x × x
= 3x^2
Second way of finding area of the bigger rectangle is adding both the rectangles' areas and subtracting by the area of the smaller rectangle.
Area of smaller rectangle:
Length × Breadth = 2 × x
= 2x
Area of bigger rectangle + Area of smaller rectangle = 3x^2 + 2x
Subtracting it by the area of the smaller rectangle:
3x^2 + 2x - 2x = 3x^2