Question

is y=0 a horizontal line

Answer 1

Yes.

Answer 2

Y=0 is a point on a graph, in order to tell if it's a horizontal line, you need a second point.

Answer 3

how about 3x+2y=o

Answer 4

y = mx + b
is the equation of a line with m slope and (0, b) as the y-intercept.
y = 0 can be thought of y = 0x + 0, with m = 0, and b = 0, so it's a line with slope = 0 that passes though (0, 0).

Answer 5

3x + 2y = 0
can be solved for y and put in the y = mx + b form:
3x + 2y = 0
2y = -3x
y = (-3/2)x
Since y = (-3/2)x = (-3/2)x + 0,
m = -3/2 and y-intercept (0, 0)