Answer
He would have to sell the stock at 38.64 dollars.
Work Step by Step
We can use the following guideline to solve this problem:
Buying price - Price drop = Selling price
So, 42.00 - 8% $\times$ 42.00 = Selling price
42.00 - .08 $\times$ 42.00 = Selling price
42.00 - 3.36 = Selling price
38.64 = Selling price
He would have to sell the stock at 38.64 dollars.