Q7...pls answer ..fast...tomorrow is my exam...no links please

Q7...pls answer ..fast...tomorrow is my exam...no links please . The mean and standard deviation of a set of ni observations are and h, respectively while the mean and standard deviation of another set of n observations are and sy respectively. Show that the standard deviation of the combined set of (n, + '12) observations is given by (ST)2 + ni112(ii — 2 S.D. =