Explanation:
9.
x = 3
yes, that's really it.
it just means that any value is valid for y.
this line is not a function, because the one valid value for x (3) has infinitely many assigned y values.
10.
the usual equation for a line is
y = ax + b
with "a" being the slope, "b" being the y-intercept (the y-value when x = 0).
we see that b = 0, as the line goes through the origin (0, 0).
the slope is the ratio
y coordinate difference / x coordinate difference
when going from one point on the line to another.
we see e.g. the points (0, 0) and (1, -3). I always recommend to look for points with integer coordinates.
when going from one to the other,
x changes by +1 (from 0 to 1).
y changes by -3 (from 0 to -3).
the slope "a" = -3/1 = -3.
the line equation is
y = -3x