Answer:
X=2
If 21+6=27 then subtract what f(x) is (which is 18) from the 27. 27-18=6 then 3x2=6 so therefore your answer is 2. Hope this helps!!
This solution might become quite complex.
Please try to keep up with me, and I'll go slowly:
You said (x + k) = (x + k - 1) (x + k)
Divide each side by (x+k): 1 = (x + k - 1)
Add 1 to each side: 2 = (x + k )
Subtract 'k' from each side: 2 - k = x