Use the given endpoint Y and the midpoint M of YZ (there is a line over YZ) to find the coordinates of the other endpoint Z

The x-coordinate of our midpoint = 3

The x-coordinate of Y = 0

k  = x-coordinate of Z

3 = (0 + k) / 2

Multiply both sides by 2

6 = k + 0

Simplify

6 = k

The y-coordinate of our midpoint = 3

the y-coordinate of Y = 5

w = y-coordinate of Z

3 = (w + 5) / 2

Multiply both sides by 2

6 = w + 5

Subtract 5 from both sides

1 = w

For problem #17, Z(6, 1)

Let's do problem 18 now.

The x-coordinate of our midpoint = 5

The x-coordinate of Y = -1

k = x-coordinate of Z

5 = (k + (-1)) / 2

Multiply both sides by 2

10 = k + (-1)

Simplify the right side

10 = k - 1

Add 1 to both sides

11 = k

The y-coordinate of our midpoint = 9

The y-coordinate of Y = -3

w = y-coordinate of Z

9 = (w + (-3)) / 2

Multiply both sides by 2

18 = w + (-3)

Simplify the right side

18 = w - 3

Add 3 to both sides

21 = w

For problem #18, Z(11, 21)