Sn=2an-3n
S_{n-1}=2a_{n-1}-3(n-1)
相减得an=2an-3n-[2a_{n-1}-3(n-1)]
an=2a_{n-1}+3
an+c=2(a_{n-1}+c)
=>c=3
an+3=2(a_{n-1}+3)
所以an+3是一等比数列
c=3