Given that,
x=a+bt+ct2+dt3
x is the displacement.
Now, by principle of homogeneity
a=bt=ct2=dt3=x
Now, a=L
[bt]=[L]
b=[LT−1]
c=[LT−2]
d=[LT−3]
Hence, the dimension of a, b, c and d is [L],[LT−1],[LT−2]and[LT−3]