remember what you can do with equations
a) multiply or divide both sides by the same thing
b) add or subtract the same thing to both sides
$\displaystyle \mbox{you have }m=\frac{x_1+x_2}{2}$
and you want to modify this so that the $\displaystyle x_2$ is by itself on one side of the = sign.
You can do this in 2 steps, the first step is (a) above, and the second step is (b)
see if you can figure out the details.