Answer: x = 27
Work Shown
rise = change in y values = x-3
run = change in x values = 1-9 = -8
slope = rise/run = -3
rise/run = -3
(x-3)/(-8) = -3
x-3 = -8*(-3)
x-3 = 24
x = 24+3
x = 27 is the answer
Let's check the answer.
The claim is the slope of the line through (9,3) and (1,27) is m = -3
The scratch work is shown below.

The answer is confirmed.