3又1/7 x (3又1/7 - 7又1/3）x 7/22 ÷1又1/21
=22/7 x7/22× (22/7 - 22/3）x 21/22
= （22/7-22/3）×21/22
=22/7×21/22-22/3×21/22
=3-7
=-4