8-9 can anyone help it’s pre algebra

In a two-digit number the units' digit is 7 more than the tens' digit. the number with digits reversed is three times as large as the sum of the original number and the two digits. find the number.

Oliver had $50 on the day before his birthday. after he recived some money for his birthday , he had $75. write and solve eqaution to find how much money oliver received for his birthday. x+50=75

Idon't get undoing if its pass adding and subtracting so can someone ? x-2 over 5 = 18