If m is the midpoint between x=a and x=b, then you have
... (a+b)/2 = m
... a+b = 2m
... b = 2m - a
For the case where a=1 and m=6, we find the reflected coordinate to be
... 2·6 - 1 = 11
Now, we want to reflect about the line x=4, so a=11 and m=4. The second reflection results in an x-coordinate of
... 2·4 - 11 = -3