We'll need the slope formula first

m = 

And then we will need point-slope formula:

y - y1 = m(x - x1)

(-2,a) and (a,3) has slope -1/2

m = 

m = 

m = 

We are give that that equals 

 = 

 = 

Cross multiply:

2(3-a) = -1(a+2)

  6-2a = -a-2
    -a = -8
     a = 8

So the points 

(-2,a) and (a,3) become the points

(-2,8) and (8,3) 

Using the point-slope form:

  y - y1 = m(x - x1)

(-2,8)

   y - 8 = (x - (-2) )

   y - 8 = (x + 2)

Multiply both sides by 2

2(y - 8) = 2×(x + 2)

 2y - 16 = -1(x + 2)

 2y - 16 = -x - 2

  x + 2y = 14

Edwin