原式= 3/4 - 7/2 - 1/6 + 2/3 - 1
=9/12 - 42/12 - 2/12 + 8/12 - 12/12
=(9 - 42 - 2 + 8 - 12 )/12
= -39/12
= -13/4 (4分之13)