`a` , `2/3` `x` `-` `1/3` `x` `=` `5/12`
`→` `(2/3-1/3)x` `=` `5/12`
`→` `1/3` `x` `=` `5/12`
`→` `x=5/12:1/3`
`→` `x=5/12` . `3`
`→` `x=5/4`
`Vậy` `x=5/4`
`b` , $3^{x}$ . $2^{x}$ `=` `216`
`→` $(3.2)^{x}$ `=` `216`
`→` $6^{x}$ `=` `216`
`→` $6^{x}$ `=` $6^{3}$
`→` `x=3`
`Vậy` `x=3`