The first discount to be applied reduces the price to 70% of the original price=0.7x, because we simply subtract 30% from 100% to get the new price. The price is further reduced to 100-15=85% of this price, so:
P(x)=0.85 * 0.7x=0.595x.
Another way to approach this is to use an intermediate function Q(x)=0.7x and then calculating P(Q(x)) where P(x)=0.85x. P(Q(x))=P o Q (x)=P(0.7x)=0.85 * 0.7x=0.595x.