A particle is moving along a straight line path according to the relation :
s2=at2+2bt+c
∴2sdsdt=2at+2b
or dsdt=at+bs, again differentiating
d2sdt2=a.s−(at+b)dsdts2=as−(at+b)(at+bs)s2
∴d2sdt2=as2−(at+b)2s3
∴d2sdt2=as−(at+b)2s3