Can someone kindly answer number 4?

X = list price = price before discount

15% is the discount, ie, we take 15% of x and subtract it off the original price

final price = (original price) - (15% of original price)
final price = x - 15% of x
final price = x - 0.15x
final price = 0.85x

The final price (or sale price) is $722.50, therefore

0.85x = 722.50
0.85x/0.85 = 722.50/0.85
x = 850

The laptop was originally $850

Final 850