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Note that the width of each rectangle   =  48 /  3  =   16 in

And the height of each rectangle  = 36 / 4  =  9 in

So.....the area of each of these rectangles  =   16  *  9   =   144 in^2

And the shaded area  is 1/2 of this =    72 in^2

The entire window is  48 x 36 = 1728 in²

    ^{made up of 24 triangles     each triangle (which is shaded region) =   1728/24 = 72 in^2}