1/2（a+2b-3c)+5(2/3a-1/2b+2/3c)-3(a-2b+c)
=1/2a+b-3/2c+10/3a-5/2b+10/3c-3a+6b-3c
=(1/2+10/3-3)a+(1-5/2+6)b+(-3/2+10/3-2)c
=5/6a+9/2b-1/6c