Много баллов! Превосходное решение.

(sin2x + cos2x)(sin2x - cos2xsin2x + cos2x) = 7/16

(sin2x + cos2x) - 3cos2xsin2x = 7/16

1 - 3cos2xsin2x = 7/16

3cos2xsin2x = 9/16

cos2x(1-cos2x) = 3/16

cos2x - cos2x = 3/16

16cos2x - 16cos2x + 3 = 0
D = 256 - 192 = 64 = 8

cos2x = (16 - 8)/32 = 1/4

cos2x = -1/2
2x = +- 5/6 + 2k, kZ
x = +- 5/12 + k, kZ

cos2x = 1/2
2x = +- /6 + 2k, kZ
x = +- /12 + k, kZ

cos2x = (16 + 8)/32 = 3/4

cos2x = -3/2
2x = +- 2/3 + 2k, kZ
x = +- /3 + k, kZ

cos2x = 3/2
2x = +- /3 + 2k, kZ
x = +- /6 + k, kZ

Ответ: +- 5/12 + k, kZ; +- /12 + k, kZ; +- /3 + k, kZ; +- /6 + k, kZ