f'(x)=6cos(3x+π/3)
3x+π/3=kπ+π/2
3x=kπ+π/2-π/3=kπ+π/6
x=kπ/3+π/18
最大值=3,
取最大值时x的集合：｛x|x=kπ/3+π/18,k∈Z}