a) In order to find the mean, consider that it is equal to the quotient between the sum of the given data, over the total number of data, just as follow:

Then, the mean is 7.2
b) Take into account that median is the value at the center of the ordered list. Then, you have:
3 5 5 7 10 13
Now, in the case in which there are two values at the center, the median is one half the sum of them:

Then, the median is 6