y=3sin(π/6-x/2)=-3sin(x/2-π/6)的单调递增区间是y=3sin(x/2-π/6)的减区间
令2kπ+π/2≥x/2-π/6≥2kπ+3π/2
4kπ+2π/3 ≥x≥4kπ+5π/3
即y=3sin(π/6-x/2)的单调递增区间是[4kπ+2π/3 ,4kπ+5π/3]