Study the given pattern carefully and select the number that can replace the question mark (?) in it.

First row - 4, 6, 13

Second row - 9, 8, 21

Third row - 5, 6, ?

(NOTE : Operations should be performed on the whole numbers, without breaking down the numbers into its constituent digits. Eg. 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)