A brokerage firm charges 6% commission on gold purchases in amounts from $50 to $3000. For purchases exceeding $3000, the firm charges 2% of the amount purchased plus a $15 handling fee. Let x denote the amount of gold purchased (in dollars), and let f (x) be the commission charge as a function of x. 

(a) Describe f (x). 

(b) Find f (3000) and f (4500).