Calculate the average acceleration (Tricky question)

Eric starts from rest and accelerates down a 32 meter long toboggan run in 6.2 seconds, without anyone pushing.

a) How big is the average acceleration? 
b) how long would the descent have taken if Isaac's father had given Isaac the starting speed 5m / s

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This is how I thought I could solve the problem.... 

v0=0m/s

s=32m 

t=6.2s 
a= delta v/delta t 

(we  have to find  v) in order to find the average acceleration .we have to use the formula s=((v+v0)/2 )*t 

But here is the tricky thing that confuses me .. Why should we put s as its negative? 

why should we write it in this way :

-32= ((0+v)/2 )*6.2 

why is it wrong to write it in this way 

32= ((0+v))/2))*6.2? 

this is what confuses me the most because in the b question I should use S as it’s positive