令bn=an-n,
a1=2,=>b1=1
a（n+1）=4an-3n+1 => a(n+1)-(n+1)=4(an-n) => b(n+1)=4bn => bn=(b1)*4^(n-1)=4^(n-1)
an=bn+n=4^(n-1)+n