if 5 times a number is decreased by 12, the difference is 9.
If you mean x - (5x -12) = 9, then
x - 5x + 12 = 9
-4x = -3
x = 3/4
If you mean (5x -12) - x = 9, then
I leave it to you to figure out what x is (it is NOT 3/4)
Please be careful when specifying the problem.