coordinates

3y + 2x  =  7 - x + 8y    rearrange as

3x - 5y  = 7

In the form    Ax + By =  C, the slope is  given by      -A/B

A = 3    B  = -5

So.....

-A / B  =  -3 / -5  = 3/5

To find  m   we have

(3/5) *m    =  -1

m = -1 (5/3)

m  =   -5/3