We know that the two trinagles are right-angled triangles so,

2,5² + a² = 4²

6,25 + a² = 16

16 – 6,25 = a²

9,75 = a²

a = √9.75

We know that the quadrilateral is a rectangle

so the opposite sides have the same length

So

8² + a² = 8,5²

16 + a² = 72,25

72.25 -16 = a²

a² = 56,25

a = √56.25

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