How do you do this logarithm problem?? Answer soon please I really need help!!!

Given log base z of 7 = 0.936 and log base z of 3 = 0.528, find log base z of 21. How do I do that?? Please answer quickly!!!!!!

Log base a of (mn) = Log base a of m + Log base a of n

Log base z of 21 = Log base z of (7 times 3)

Log base z of (7 times 3) = Log base z of 7 + Log base z of 3

Log base z of 7 + Log base z of 3 = 0.936 + 0.528