Question 1045525
{{{drawing(300,300,  -10,10,-10,10,   grid(1), 
blue(line(1,-3,-3,-1)) ,
circle(-3, 8,0.3),
circle(-1,7,0.3),
blue(line(1,-3,3,5)) ,
blue(line(1,-3,3,5)) ,
green (line(3,5,-3,8)) ,
green(line(-3,-1,-1,7)) ,
graph( 300, 300,-10,10,-10,10,  0  ))}}}
m ={{{(y[1] - y[2])/(x[1] - x[2] )}}}
(-3,-1)
( 1, -3)  m = 2/-4 = 1/-2 = 3/-6 or P(-3,8) which is back 6 & up 3 from P(3.5)
(1,-3),  
(3,5),   m =  -8/-2 = 4 = 8/2  or P(-1,7) which is up 2 & up 8 from P( -3,-1) 

P(-1,7) the fourth vertex
Using graph paper one could also sketch the green lines to fing the fouth vertes P(-1,7)