Answer:
(-3, 3)
Explanation:
Given:
Midpoint = (6, 10)
Endpoint 1 = (15, 17)
The midpoint formula:
M(x, y) =

Where (x1, y1) represent one pair of coordinates and (x2, y2) represent the other pair. The (x, y) in the beginning stands for the x - value of the midpoint and y - value of the midpoint.
So to solve for a missing endpoint given the midpoint and the other endpoint, we just have to apply this formula.
Let's plug in what we have into this.
Since we know the x and y values of the midpoint (6, 10), we'll make two equations setting one to x(6) and the other to y(10).
Then, we substitute one of the x-values (in the numerator) for the first equation to the x - value of the given endpoint(15) and solve for x. For the second equation, we set one of the y-values(in the numerator) to the y - value of the given endpoint(17) and solve for y.
Let's set these equations up,
First let's solve for the x - value of the endpoint:
(we substitute one x - value in the numerator for the x - value of the given endpoint which is 15, then we set the whole equation to equal 6 since that is the midpoint's x - value and we are trying to solve for the x - value of the endpoint.)

(we are basically averaging)
Multiply both sides by 2 (inverse property of division is multiplication):
15 + x2 = 12
Subtract 15 from both sides (inverse property of addition is subtraction):
x2 = -3
The endpoint's x - value is -3.
Now let's solve for the y - value of the endpoint:
(we substitute one y - value in the numerator for the y - value of the given endpoint which is 17, then we set the whole equation to equal 10 since that is the midpoint's y - value and we are trying to solve for the y - value of the endpoint.)

Multiply both sides by 2 (inverse property of division is multiplication):
17 + y2 = 20
Subtract 17 from both sides (inverse property of addition is subtraction):
y2 = 3
The endpoint's y - value is 3.
Now, let's put what we found together in a coordinate format:
(x, y)
= (-3, 3)
The other endpoint is (-3, 3).