In each of the given number-clusters, the number on the right side of (the equal to sign) is calculated by performing certain mathematical operations on the three numbers on the left of (the equal to sign). All three number-clusters follow the same pattern. Select the number from among the given options that can replace the question mark (?) in the third number-cluster. 15, 8, 12 = 10 26, 6, 13 = 12 11, 12, 4 = ?