Hope you can follow the many steps in the working.

Step 1: List the flow of events in black

Step 2: Jack left with 4/7 of his marbles in the 2nd Change. So he was given 4 units and Peter also had 4 units left (Blue). (Same number at the end)

Step 3: Working backwards - Jack should have 7 units before he gave 3 units (Orange) to Peter. So, Peter had 3 less than 4 units = 1 unit (Purple) before 2nd Change.

Step 4 : During 1st Change, Peter gave 1and1/2 units to Jack and had 1 unit (Purple) left

Step 5: At first, Peter had 2 and 1/2 units and Jack had 5 and 1/2 units. In total they had 8 units which was shown in the working below.