The FWs should be placed in-line, so that it can see traffic from the campus LAN, to traffic on the data center.
Here is a link to give you some best practices and may answer some questions.
cisco campus lan design ( access distribution core )
In data center we are using cisco vpc (spine and leaf )
Now I want to place an (acive standby fw ) in data center .
I place the Please guide me to do so , Where should I Place the firewall
Given you mentioned "spine and leaf" I'm going to assume you've got an ACI deployment. Like @SteveCantwell mentioned the FW needs to be inline. The natural spot would possibly be plugged into your core routing device which would be connected to your ACI fabric, but ultimately where it would get plugged in will vary based on your specific network design and needs.
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