Final answer:
For matrix AB to be defined, the number of columns in matrix A must be equal to the number of rows in matrix B. Since A is a 2 x 6 matrix, m must be 6.
Step-by-step explanation:
The question pertains to matrix multiplication. For the product of two matrices to be defined, the number of columns of the first matrix must be equal to the number of rows of the second matrix. Given that A is a 2 x 6 matrix and matrix AB is defined, matrix B must have 6 rows to comply with the rules of matrix multiplication; hence, the value of m, representing the number of rows in matrix B, is 6.