Kinematics example (Ari's set)

Hi, can somebody explain how to get to the answers? a) 112 s, b) 4480 m.
I tried 2 methods but got different results each time. Thanks!

Sorry to bother with this topic again, but there is something unclear to me still. We already added +50 seconds in the beginning when we defined s1. Why do we need to add 50s again, wouldn’t that make it a total of 100s instead of 50? Let me know if my question is clear enough