Find the slope of the line that passes through the points (-10,-9)and (-10,8).

The slope/gradient formula is
(y₂ - y₁) = m(x₂ - x₁)
Plug in the coordinates to find the slope
(-10,-9) and (-10,8)
y₂ = 8                      y₁ = -9
x₂ = -10                   x₁ = -10

(8 -(-9) = m(-10-(-10)
(8 + 9) = m(-10 + 10)
(17) = m(0)
divide by 0
= m
Now, this is a math error, therefore, we cannot find the slope which has the same x - value, which, in this example is, -10

The answer is part a: use