You can use an "array formula" like this
=INDEX('Sheet2'!B:B,MATCH(1,(A1='Sheet2'!A:A)*(C1='Sheet2'!C:C),0))
CTRL+SHIFT+ENTER
....or you can add another INDEX function so that it doesn't need to be "array entered", i.e.
=INDEX('Sheet2'!B:B,MATCH(1,INDEX((A1='Sheet2'!A:A)*(C1='Sheet2'!C:C),0),0))
or another way is to use LOOKUP like this
=LOOKUP(2,1/(A1='Sheet2'!A:A)/(C1='Sheet2'!C:C),'Sheet2'!B:B)
That latter method would give you the last match if there is more than one......
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