These problems are so much easier to write out if you do some trig to begin with. Here the 3 and the 4 are a bit of a giveaway indicating a 3,4,5 triangle. Taking the liberty of changing the d to the more usual s,
where
That doesn't get you pass the calculus however.
Velocity
Acceleration
The particle will be at rest when the velocity is zero, that is, when etc.
seems you are in way over your head ... recommend you visit the links
Trigonometry | Khan Academy Video Course
Calculus | Khan Academy Video Course
In the end you will just understand this question, but when thrown another question similar to this you will get it wrong..It's ok I think I may be close to understanding.
Trust me I am talking from personal experience, it never pays.
You should watch those Khan videos, the guy is brilliant.
Here is another link to the physics Section:
Khan Academy