How to find third coordinate when given midpoint and a point?

Example : 
u have an endpoint at (6,5)ad the midpoint is (4,2)find the other endpoint.

midpoint formula : (x1 + x2) / 2, (y1 + y2) / 2
one endpoint (6,5)x1 = 6 and y1 = 5
other endpoint (x,y)...x2 = x and y2 = y
sub into the formula
m = (6 + x) / 2, (5 + y) / 2

okay, so the midpoint is (4,2)so ur x value will equal 4
(6 + x) / 2 = 4...multiply both sides by 2
6 + x = 4 * 2
6 + x = 8
x = 8 - 6
x = 2 <==

midpoint is (4,2)so the y value will equal 2
(5 + y) / 2 = 2 ...multiply both sides by 2
5 + y = 2 * 2
5 + y = 4
y = 4 - 5
y = -1 <==

so ur other endpoints are (2,-1)

Most of the people don’t make the sense of taking it