How do you do Test 9 section 3 number 20?
This question is about perpendicular lines, so the first thing to remember is that perpendicular lines have negative reciprocal slopes. The graph of 
, shown, has a slope of -2. That means the slope of 
, not shown, must have a slope of 
.
The second thing to quickly note is that this question is using function notation and asking for 
. That’s the same as asking for the 
-intercept; it’s just the question 20 way to ask it.
We know that passes through 
, and we know it has a slope of , so all we really need to do is count leftwards. is only 1 unit away from the -axis, and when you have a slope of , you travel 0.5 units down when you travel 1 unit left. Therefore, the -intercept—aka —is 2.5.
If you like algebra better, you can plug the slope and the point you know into slope-intercept form (
) and solve for 
:
