ลูกค้าชื่อ Carole และนามสกุล Bernhard กลายเป็นลูกค้าเมื่อใด
When did customer with first name as Carole and last name as Bernhard became a customer?
df_customers = pd.DataFrame(columns=['date_became_customer', 'first_name', 'last_name'])
df_customers[(df_customers['first_name'] == 'Carole') & (df_customers['last_name'] == 'Bernhard')]['date_became_customer']