The area of the whole rectangle was 2 x 4 = 8 until you cut that right triangle off the corner.
The legs of that right triangle are 2 – 1 = 1 and 4 – 2 = 2
Area of a triangle is (1/2)(base)(height) = (1/2)(1)(2) = 1
So the area of the shape is 8 – 1 = 7 units2
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