Line cd contains points a (4, −7) and b (4, 8). the slope of line cd is

Given two points; A(x₁,y₁) and B(x₂,y₂) the slope "m" is:
m=(y₂-y₂)/(x₂-x₁)   or   m=(y₁-y₂)/(x₁-x₂)

In this case:
A(4,-7)
B(4,8)

m=(-7-8) / (4-4)=-15/0  because we have "0" in the denominator the line is vertical, and its slope is indefinitely. 

Basic algebra equation. first set up your equation: divide out 9.and now you are left with 2x + 4 = 8.