Since Arnold has 75% more money saved, then he has 175% of the money he had in March. This is equal to:

He has $525 saved.
The guitar is on sale for a discount of 30%, so the new price is 70% of what it used to be. The new price is:

But he also needs to pay the sales tax of 5%, which will make the total paid equal to 105% after the discount. We have:

Since the amount of money Arnold has saved is greater than the price of the guitar after the disccount and tax, he has enough money to buy it.