Validation rule with a lookup

Hi,

I'm wondering if somebody could help with this please? This is on a custom object: Payment. There are two fields: 'BACS' which is a lookup to BACS records and a picklist 'Payment Details Confirmed?' which has three values in.

I would like a validation where a user can only choose/save a value in the 'BACS' field if one of the three values has been selected in the  'Payment Details Confirmed?' 

Many thanks, Rob 

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