To find the sale price for Karen's haircut, we need to apply the 50% discount to the original price of $84.
Step 1: Calculate the amount of discount.
Discount = 50% of $84
Discount = (50/100) * $84
Discount = $42
Step 2: Subtract the discount from the original price.
Sale Price = Original Price - Discount
Sale Price = $84 - $42
Sale Price = $42
Therefore, the sale price for Karen's haircut, after applying the 50% off coupon, is $42.