For the details of the effects, Section 12.22 & Section 12.23
(a) The order of acidity is explained by `-I` effect of the chlorine atoms. Greater the magnitude of `-I` effect, easier will be the release of `H^(+)` from `O-H` bond and stringer will be acid.
(b) The order of acidity is explained by `+ I` effect of alkyl groups. As the number of alkyl groups increases, the magnitude of `+ I` effect also increases. As a result, the release of `H^(+)` from `O-H` bond becomes more and more difficult and acidic strength decreases.