Question 1025344
The distance between those two points is {{{1-(-8)=9}}} so then the missing distance is,
{{{P=2x+2y=26}}}
{{{2(9)+2y=26}}}
{{{18+2y=26}}}
{{{2y=8}}}
{{{y=4}}}
The y coordinate can either move up or down,
{{{y[1]=-6-4=-10}}}
or 
{{{y[2]=-6+4=-2}}}
So the other two points are either,
(1,-10)
(-8,-10)
or
(1,-2)
(-8,-2)