The missing coordinate r is 11.
Solution:
Given points are (–15, 1) and (–7, r).

Slope (m) = 
<u>To find the missing coordinate r:</u>
Slope formula:

Substitute the given values in the formula.



Do cross multiplication.

40 = 4r – 4
Add 4 on both side of the equation.
44 = 4r
Divide by 4 on both side of the equation.
11 = r
⇒ r = 11
Hence the missing coordinate r is 11.
Answer:
![\left[\begin{array}{ccc}-32\\4\\20\\-36\end{array}\right]](https://tex.z-dn.net/?f=%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7D-32%5C%5C4%5C%5C20%5C%5C-36%5Cend%7Barray%7D%5Cright%5D)
Step-by-step explanation:
The product of a scalar times the matrix involves the product of that scalar times each of the elements of the matrix, resulting on the multiplication by "-4" of each of the four elements as shown below. These new product values end up being the new matrix elements.
![-4*\left[\begin{array}{ccc}8\\-1\\-5\\9\end{array}\right] = \left[\begin{array}{ccc}-32\\4\\20\\-36\end{array}\right]](https://tex.z-dn.net/?f=-4%2A%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7D8%5C%5C-1%5C%5C-5%5C%5C9%5Cend%7Barray%7D%5Cright%5D%20%3D%20%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7D-32%5C%5C4%5C%5C20%5C%5C-36%5Cend%7Barray%7D%5Cright%5D)