Translation:The convenience store is west of the bank.
You could, just the part of the sentence being stressed would change. In the context of giving directions somewhere though, は makes the most sense.
は as the topic marker stresses everything that comes after it. Once it has been introduced it can be dropped from later sentences since it is already understood information. "(As for the convenience store) it is west of the bank"
It answers the question "Where is the convenience store?"
が as the subject marker stresses what comes before it. "It is the convenience store (that is west of the bank)"
It answers the question "What is west of the bank?"
私はアメリカ人です - "(On the topic of me,) I am American"
私がアメリカ人です - "I am (the one who is) American"