2-3 1/7*(2-1 9/11)разделить на 8/21
2-3 1/7*(2-1 9/11):8/21 = 2-3 1/7*2/11:8/21 = 2-22/7*2/11:8/21 = 2-4/7*21/8 = 2-3/2 = 1/2