PGT
I suppose you have a point, in that we are trying to find indicated altitude from true and this calculation actually corrects indicated altitude to true, and agree it's a bad question and that he therefore may not be able to follow. However the only way more accurate than what I am suggesting is to use iteration, using the new calculation of indicated altitude to recalculate the correction.
I don't think pressure altitude is a good way of looking at this, because it has a weaker relationship with indicated altitude than true altitude does, QNH having a greater effect than temperature change. If the PA had been, for example 2,500 feet instead of 3,500 feet this would have been even further out than any of the answers yet suggested, and in the low sense.
How about 18 x 4 x 3.233 = 233 feet as the correction, giving 3,233 feet as the answer from the formula to the nearest foot!
DA
Well PGT and I are training providers, so should be as good as any, and in my humble opinion we are better than most
!