If x = 12, then plugging "12" in for "x" in the original equation should work. Let's see:
. . . . .3([12] - 5) ?=? [12] + 9
. . . . .3(7) ?=? 21
. . . . .21 ?=? 21
This certainly looks like a valid solution to me. Checking over your work, all your steps look perfectly correct.
I have no idea what your instructor might be getting at, but, if the problem is as posted, you're right.
Eliz.