We will use the formula for the angle q between two lines,

tan(q) = , where q = 135° and tan(135°) = -1

First we must find the two slopes

Let m2 = the slope of the green line, and m1 = the slope of the red line

To find the slopes we use the slope formula:

m2 = 
where (x1,y1) = (-1,y)
and (x2,y2) = (4,-7)

m2 =  =  =  =  = 

and

m1 = 
where (x1,y1) = (4,2)
and (x2,y2) = (-1,-9)

m1 =  =  =  = 

And we substitute in the formula for the angle between two
lines:

tan(q) = 

tan(135°) = 

-1 = 

Multiply both sides by the denominator on the right:

 = 

 = 

Multiply through by 25

-25+(77+11y) = 55+5(7+y)
  -25+77+11y = 55+35+5y
      52+11y = 90+5y
          6y = 38
           y = 
           y = 

Edwin