Finding out missing observation 
The Standard deviation of two observation is $2$, and one of the observation is $7$, find the other observation

I have no idea how to begin with this problem.
If mean was given then it would be easy , but i have no idea how to solve it with only standard deviation given

Hint: Suppose the observations are $7$ and $x$. The mean is $\mu =\frac{7+x}{2}$. Now can you write an expression for the standard deviation in terms of the observations ($7$ and $x$) and the mean $\mu$ (which we just showed is $\frac{7+x}{2}$) and set it equal to $2$?
