# Thread: Halp! Linear equation questions.

1. ## Halp! Linear equation questions.

I'm supposed to find the y-intercept, slope, and equation for a couple problems. I'm having a lot of trouble with these. This first one has the two points on a graph (0,3) and (6,6) The y intercept is 3 and the slope is 3, but how do I find the equation? The second one has the points (2,18) and (6,6). the slope is -3 (right?) but how do you find the y-intercept when the x-axis skips 0? How do you find the equation for that? The third one has no y intercept because X stays the same. The points are (6,3) and (6,10). There is no slope and no y-intercept, so how do I say y goes up by 7 in an equation?
Thank you SO much.

2. A line's equation can be either y = mx + c or x = k.

If x1 != x2 then m = (y2 - y1) / (x2 - x1) = slope and c = y-intercept
when given two points on the line as (x1, y1) and (x2, y2).
Substiture in first equation, y = 3x + 3 is the line equation.

Similarly for the second, x1 = 2, y1 = 18, x2 = 6, y2 = 6 so
m = (6 - 18) / (6 - 0) = -2
so y = -2x + c, use any one of the points, say (0, 18) to put
y = 18 = -2x + c = -2 * 0 + c = c
equation is y = -2x + 18

3rd equation: x1 = x2 so line equation is x = k, using any point, x = 6.

3. Originally Posted by hwhelper
3rd equation: x1 = x2 so line equation is x = k, using any point, x = 6.
Thanks, but what should I say if I want to say y= something?