Help maths

The integral or the area between a velocity-time graph corresponds to displacement (gcse physics), as velocity is the rate of change of displacement. So t1 corresponds to the first time that the area above (positive displacement) and the area below (negative displacement) the x-axis are equal and so it corresponds to a zero displacement at that time.

The velocity is zero at 5 and 12, not displacement.

Velocity-time graphs
https://www.bbc.co.uk/bitesize/guides/z3whb82/revision/4#:~:text=For%20a%20moving%20object%2C%20the,that%20the%20object%20is%20accelerating.
Initially the velocity is negative so the resulting displacement is negative. For t>5 the velocity changes sign to being positive so particle will be heading back towards the initial position. This occurs when t=8 and the positive displacement (t>5) "equals" the negative displacement(t<5) or equivalently the two corresponding areas on the velocity time graph are equal