Perfecto! The stone is going up, so, the \(s\) is positive. \(a\) is always negative as the Earth is pulling the object down. Also, when the stone reaches its maximum height, the final velocity \(v = 0\)
Hopefully, these examples have shown how you can use the formulae to calculate unknown quantities. The first step is to always write down the quantities that you know, and those that are being asked. Then, you need to identify the correct EOM for these quantities. And, that's it!