midpoint

Let ( x, y)  be  the coordinates of Q

Using the  mid-point formula  we have

x coordinate of P + x coordinate of Q 

_________________________________   =   x coordinate of  M

                     2

[ 11 + x ]        =       5           multiply both sides by 2

_______

      2

11 +  x  = 10     subtract  11 from both sides

x  =  -1

And  

 y coordinate of P  +  y coordinate of Q     =  y coordinate of M

_________________________________

                      2

[ -10  +  y  ]   =   -  2                multiply both sides by  2

___________

        2

-10 +   y  =  -4          add 10 to both sides

y   = 6

So.... Q =  ( x, y  )  =  ( -1 , 6 )