From the end R to the midpoint M the x co-ordinate goes from 6 to 0. So to carry on going to the other end, it has to go as far again as it's already gone, that is, from 0 to -6.

Similarly with the y co-ordinate, it goes from 0 to 2, so it has to go as far again, that is, from 2 to 4.

It doesn't really need a formula, just thinking in the right direction (no pun intended).