how do i find the other endpoint if I have an endpoint of 12, -8 and a midpoint of 3,18?

Answer 1

Mid-points are the average of the two end-points
You have one of the end points, namely(12,-8)
Let the other one be (x,y)
(12+x)/2=3
12+x=6
x=6-12
x=-6

(-8+y)/2=18
-8+y=36
y=36+8
y=44
Those are your end-point coordinates: (-6,44)

Answer 2

Given endpoint A(12, -8) and midpoint M(3, 18), find the other endpoint B.

Calculate the directional vector AM.

AM = = <3-12, 18+8> = <-9, 26>

Now move from M and additional directed distance of AM.

B = M + AM = <3, 18> + <-9, 26> = <-6, 44>

The other endpoint is B(-6, 44).