Answer:

Step-by-step explanation:
We know that the slope of the line is -1. 
We also know that the line passes through (-9, 3) and (-10, r). 
We want to find the value of r. 
First, let's figure out the equation of our line. We can use the point-slope form: 

Where m is the slope and (x₁, y₁) is a point. 
So, let's substitute -1 for m. Since we know the point (-9, 3), let's use this for (x₁, y₁). 
Substitute: 

Simplify: 

Distribute: 

Add 3 to both sides. So, our equation is: 

It passes through (-10, r) and we want to find the value of r. So, let's substitute -10 for x and r for y, since -10 is our x and r is our y. So: 

Evaluate for r. Distribute: 

Subtract: 

So, the value of r is 4. 
And we're done!