We know how to find the midpoint coordinates by the following:
(a + b) / 2 = c, where a and b are the endpoints and c is the midpoint.
Well, in this case, instead of a and b and finding c, we have a and c, and need to find b. First the x-coordinate:
(3 + b) / 2 = 6
multiply both sides by 2:
3 + b = 12
Subtract both sides by 3:
b = 9
Now the y- coordinate. Follow the same rules as before, start by multiplying by 2:
(3 + b) / 2 = 9
3 + b = 18
b = 15
So our missing endpoint is (9, 15).