Measure Based on a filter from a different table

Hi,

I have a Transcations tables with "Ammount" column and "Customer ID" column for each customer's transactions.

I also have another Customers table with "Customer ID". Transactions are currenly not only showing my customers but showing also other custoumers as well.

When I use cards or graphs with, the ammount value is showing everything not just for my customers. 

Can anyone help me on creating a measure that will sum all amounts but would filter the Transcations[Customer ID] with values from table Customers[Customer ID] so that the sum value is for just my customers.