First, let's calculate the store's selling price after marking up the swing set by 180%:
Selling price = Cost price + Mark up
Selling price = $90 + (180% of $90)
Selling price = $90 + $162
Selling price = $252
So the store's selling price for the swing set was $252.
Next, the store marked it down by 50%, which means the new selling price is:
New selling price = Selling price - (50% of Selling price)
New selling price = $252 - (50% of $252)
New selling price = $252 - $126
New selling price = $126
Therefore, the final discount price of the swing set is $126.