$a)CH_2=C(CH_3)-CH=CH_2+Br_2$$\rightarrow CH_2Br-C(CH_3)=CH-CH_2Br$
$\rightarrow CH_2Br-C(CH_3)Br-CH=CH_2$
$\rightarrow CH_2=C(CH_3)-CHBr-CH_2Br$
$b)C_6H_5OH+3Br_2\rightarrow C_6H_2Br_3OH\downarrow+3HBr$
$C_6H_5-CH=CH_2+Br_2\overset{CCl_4}{\rightarrow}C_6H_5-CHBr-CH_2Br$
$c)C_6H_5OH+NaOH\rightarrow C_6H_5ONa+H_2O$
$C_6H_5ONa+CO_2+H_2O\rightarrow C_6H_5OH+NaHCO_3$