answer: When we are given 2 points, we can find the slope using those 2 points. Now one point is missing a y-coordinate. We use the formula for slope to find the value of r.
Slope = (y2 - y1) / (x2 - x1)
Let Point 1 = (-9, r) = (x1 , y1)
Let Point 2 = (-8, 0) = (x2 , y2)
Plug in these values into the formula for slope. By doing so, we can get an equation that allow us to solve for r.
6 = (0 - r) / (-8 - (-9))
6 = -r / (-8 + 9)
6 = -r / 1
Multiply both sides of equation by 1.
6 = -r
Divide both sides of equation by -1.
-6 = r
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