State I: tells you that S is +ve and nothing more - so its insufficient.

State II: tells you the distance as follows:

t - r = t - (-s)

we dont know if r is positive or negative so therefore we cannot conclude anything about the distance between t and r

Combining two statements:

s is to the right of 0, therefore s is a positive number
distance between t - (-s) = t + s = t - r

t + s - t + r = 0
s + r = 0

which means s and r equal and of opposite signs.

Therefore, 0 is between s and r