`-` Their zoo hasn't welcomed a baby elephant for five weeks .

`-` It's five weeks since their zoo last welcomed a baby elephant.

`-` The last time their zoo welcomed a baby elephant was five weeks ago. 

`××××××××××`
`+` S + haven't/hasn't + PII

`=` S + last + Ved....

`=` The last time S + Ved + was + thời gian + ago\

`=` It's + thời gian + since + S + (last) + Ved 

Answer :

`->`Their zoo hasn't welcomed a baby elephant for five weeks.

`->`The last time their zoo welcomed a baby elephant was five weeks ago.

`->`It's five weeks since their zoo last welcomed a baby elephant.

`@`Form :

S + last + V2/ed + time ( thời gian trong quá khứ )

`=`S + have/has not + Vpp + since/for + time

`=`It’s + time + since + S ( last ) + V2/ed

`=`The last time + S + V2/ed + was + time