Answer:
r = 11
Explanation:
Since we have a slope and one confirmed point, a good approach would be to find the equation of the line that goes through that point. The equation can then be used to find r, for x = 3.
The standard format for an equation of a straight line is y=mx+b, where m is the slope and b the y-intercept (the value of y when x=0). The slope is -3:
y = -3x + b
To find b, enter the given point and solve for b:
-1 = -3*(7) + b [for (7,-1)]
b = 20
The equation for this line is y = -3x + 20
Now enter (3,r)
y = -3x + 20
r = -3(3) + 20
r = 11
The point (3,r) is (3,11)
See the attached graph