24
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4 x 6 = 24
30 square units !
10
In a 3x3 grid, you can form rectangles by choosing two horizontal and two vertical lines. A 3x3 grid has 4 horizontal lines and 4 vertical lines. The number of ways to choose 2 lines from 4 is given by the combination formula ( C(4, 2) ), which equals 6. Therefore, the total number of rectangles is ( 6 \times 6 = 36 ).
A 7-by-9 grid has an area of 63 square units.