Need to know what x is

For convenience let the sides of the square be of length 2.  Then:

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Alternatively,  since the angles of the equillateral triangle are 60°, the apex angle of each of the two large isosceles triangles is 90° - 60° = 30°, so the other two angles in the isosceles triangles are each (180°- 30°)/2 = 75°, which gives x as 90° - 75° = 15°

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Thanks a lot!