Calculating "pooled and unpooled" standard error Data given:

Treatment          Y^bar         n                 s 

1                  18.1          9                 3.851

2                  20.9          23                9.854


a. Calculate the unpooled standard error for the difference of means.

b. Calculate pooled standard error for the difference of means. 

how to do that? what do they mean by "pooled and unpooled?

Appreciate your help!
Do you know the difference between pooled and unpooled? 

Hint: pooled tests assumes that two populations have equal variances. Use this to calculate standard error.
Do you know the difference between pooled and unpooled? 

Hint: pooled tests assumes that two populations have equal variances. Use this to calculate standard error.