What is the y-intercept?

qwe12 wrote:my way of solving this is as follows, if y = mx + c, is the equation of a line, where c is the y intercept.

from 1) m = 3c

=> y = 3cx + c, to determine the y intercept, plug y = 0

0 = c(3x + 1)

Here, we can 2 options c=0 and x can be anything
or 3x+1 = 0 and c can be anything.

NOT SUFFICIENT.

from 2)

y = m(-1/3) + c
y = -m/3 + c

so c = y + m/3, if y goes to 0, c = m/3, and m can be anything,

NOT SUFFICIENT.

Using (1) and (2) we find we have 2 equations and 3 unknowns.

y = 3cx + c
y = -m/3 + c

We need 3 equations to solve for 3 unknowns.

SO (E)

To determine the y intercept, plug y = 0 . Why? Is dere any rule relating to this?