7.
The standard form: Ax + By = C



8.
The formula of a slope:

a) We have the points (-4, 7) and (8, -9). Substitute:

b) We have the points (-2, 7) and (-2, 5). Substitute:
division by 0!
slope is undefined
9.
a) The equation of a horizontal line: y = a (a - any real number)
We have the point (-5, 8) → x = -5 and y = 8
Answer: y = 8
b) The equation of a vertical line: x = a (a - any real number)
We have the point (-3, -2) → x = -3 and y = -2
Answer: x = -3