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when you're doing a perpendicular line, you want the exact opposite of your original.
take, for example, a line with a slope of 2. the line perpendicular to it would have a slope of -2.
With a fraction, its pretty tricky. Let's say you have 2/7 as your slope. you want the EXACT opposite so
First, make it negative
Next, flip it around.
You get -7/2 as your slope perpendicular to your original line.
For the point passing through, if x equals 0, y equals your y intercept. your y value is 3 therefore b (y-intercept) equals 3.
For questions that are less simple than a plain y-intercept, (which you are guaranteed to encounter in the future) you simply input the y and x values into your y=mx+b equation.
Say we have an example here of a line with a slope of 4, and it passes through (5,-3). We have to find our B, or y-intercept, so we input the values.
-3=4(5)+b
Now, simplify
-3=20+b
-23=b
Your example equation is now y=4x-23
I hope this helped! please let me know if you have any questions! :)