Answer:
First you can just flip the line on point C to make a 90 degree angle. That's the simple way.
Other way is to get the slope of AB let's say ex : -1/2
We need to get the slope of C to be a reversed reciprocal of -1/2. So, 2.
Basically I flipped -1/2 in to -2/1 then removed the negative sign for 2.
:)
Hope that makes sense to ya
