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

Where is the problem

First step is to get rid of the square root. remember that a square root is the same as raising to the (1/2) power. see below: multiply exponents per exponent rules to get: remember you subtract exponents from the denominator and add exponents in the numerator so: simplify further: so a is the correct choice. if you don't understand this google exponent rules and it will you understand. look at the image results.