Explanation:
part A:12+7x
first day 12+7=$19
second day 19+7=$26
third day 26+7=$33
....
so
number of moneys on x day will be
12+7*x where x represent the number of days
so
for three days
12+7*3=12+21=33
part B:138
12+7x where x=18
12+7*18=12+126=$138
He will have $138 on the 18th day.