Question

CuO->x1->x2->x3->CuO

Answer 1

$CuO + H_2 =^t Cu + H_2O\\Cu + 4HNO_3(concentrated ) = Cu(NO_3)_2 + 2NO_2 + 2H_2O\\Cu(NO_3)_2 + 2NaOH = Cu(OH)_2 + 2NaNO_3\\Cu(OH)_2 =^t CuO + H_2O$

$x_1 = Cu\\x_2 = Cu(NO_3)_2\\x_3 = Cu(OH)_2$