How much power did the study have for detecting a significant difference between groups (using a two-sided test with a 5% level of significance) if the true effect of the SMI intervention is to reduce mean DBP by 2 mm Hg more than the control group and the standard deviation of change within a group is 6 mm Hg? 

The results for the control group (n = 320) at the end of the study were as follows:

Mean (change) = −4.77 mm Hg,

sd (change) = 6.09 mm Hg.