The triangle on the left forms a 3 - 4 - 5 right triangle
Note that the area of this triangle = (1/2) (3)(4) = 6
And drawing a segment from the lower left vertex of the large rectangle perpendicular to the side of the shaded rectangle will also form an altitude of this right triangle
So
(1/2) (base) (altitude) = 6
(1/2) (5) (altitude) = 6
(5/2) ( altitude) = 6
altitude = 12/5 = 2.4
And this perpendicular also foms a right triangle with a hypotenuse of 3 and one leg = 2.4
So....the other leg = sqrt (3^2 - 2.4^2 ) = 1.8
And the area of this triangle = (1/2) (product of the leg lengths) =
(1/2) (2.4)(1.8) = 2.16 units^2
And, by AAS, this triangle is congruent to the smaller right triangle at the lower right of the figure
And we have two of these smaller right triangles and two of the larger right triangles
So....their total area = 2 ( 6 + 2.16) = 2 ( 8.16) = 16. 32
And the area of the large rectangle = 6 * 4 = 24
So.....the shaded area = 24 - 16.32 = 7.68 units^2
