Standard Deviation question?

I have the following problem that I'm having great difficulty solving:

One group of students (sample size **a**) scored an average of **b** on an exam. The standard deviation of these results was **c**. Another group of students (sample size **x**) scored an average of **y** in the same exam, with a standard deviation of **z**. What is the mean and the standard deviation of the two groups of students as a whole?

I know that the weighted mean is given by (**a**.**b**+**x**.**y**)/(**a**+**x**); but I have no idea how to get the weighted standard deviation. Any ideas?