The points 6,4 and 9,r lie on a line with slope 3. Find the missing coordinate r.

The points 6,4 and 9,r lie on a line with slope 3. Find the missing coordinate r.

Slope = (y2 - y1)/(x2 - x1)

Plugging in our numbers, we get:
3 = (r - 4)/(9 - 6)
3 = (r - 4)/3

Cross multiply:
r - 4 = 9

Add 4 to each side:
r = 13