Study the given pattern carefully and select the number that can replace the question mark (?) in it.
First row:7, 9, 39
Second row:4, 8, 28
Third row:6, 7, ?
(NOTE: Operations should be performed on the whole numbers, without breaking down the number into its constituent digits. For example, 13 - Operations on 13 such as adding/deleting/multiplying etc. to 13 can be performed. Breaking down 13 into 1 and 3 and then performing mathematical operations on 1 and 3 is not allowed.)