How many donations were paid via credit card to Memphis City School District?
paid via credit card refer to payment method = creditcard; Memphis City School District refer to school_district
SELECT COUNT(T1.projectid) FROM donations AS T1 INNER JOIN projects AS T2 ON T1.projectid = T2.projectid WHERE T1.payment_method = 'creditcard' AND T2.school_district = 'Memphis City School District'