You can imagine this irregular shape is cut into smaller, easier to solve for shapes. Then add up the area's of the smaller rectangles.
Or you could find the area of the whole rectangle and then subtract that cut out piece. I think this is the quickest strategy.
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If this thing is one big rectangle 20 × 8 = 160
but it's got a piece cut out that is 6 × 4 = 24
Total area = 160 - 24 = 136
*To find the missing side of the cut-out piece. Subtract the top lengths from the full bottom length. 20 - 10 - 6 = 4