Bunuel
What is the x intercept of non-horizontal line m?
(1) The slope of m is 4 times the y-intercept of m
(2) The y intercept of line m is -2.
Target question: What is the x-intercept of non-horizontal line m?Awesome question!!
Statement 1: The slope of m is 4 times the y intercept of m Let's let
y = Mx + b be the equation of line m, where M = the slope of line m, and b = the y-intercept of a line m.
Since
b is the y-intercept of line m, then the statement that tells us that
4b is the slope of line m.
So, the equation of line m becomes:
y = 4bx + bImportant concept #1: The x-intercept of a line is the x-value that corresponds to y = 0 (since y = 0 for all points on the x-axis) So, to find the x-intercept, we'll replace y with 0 and solve the following equation for x:
0 = 4bx + bFactor the right side:
0 = b(4x + 1)So, EITHER
b = 0 OR
4x + 1 = 0.
Important concept #2: Since the question tells us that line m is NOT horizontal, we know that the slope of the line cannot be zero.
In the equation y = 4bx + b, the slope of the line is 4b, which means 4b cannot equal 0, which means b cannot equal 0.
So, it must be the case that:
4x + 1 = 0Solve for x by subtracting 1 from both sides:
4x = -1Solve :
x = -1/4, which means the answer to the target question is
the x-intercept of line m is -1.4[Note: On test day, I wouldn't waste time solving 4x + 1 = 0, since it's clear that the equation will have only one possible solution]Since we can answer the
target question with certainty, statement 1 is SUFFICIENT
Statement 2: The y-intercept of line m is -2.All this tells us is that line m passes through the point (0, -2)
There are infinitely-many lines that can be drawn through the point (0, -2)
For example, line m could look like this....

...in which case the answer to the target question is
the x-intercept of line m is 1Conversely, line m could look like this....

...in which case the answer to the target question is
the x-intercept of line m is -3Since we can’t answer the
target question with certainty, statement 2 is NOT SUFFICIENT
Answer: A
Cheers,
Brent