I believe those are matrices. Please specify the topic qhen you ask a question to make things less complicated
Now, AB = C A is a 1×3 matrix ( row -column) C is a 1×2 matrix. Hence, B must be 3× 2 matrix. This is obtained from rule, when you multiply 2 matrices , number of columns of first matrix must equal to no of rows of second matrix. The product (C) will have number of rows equal to number of rows of first matrix(A) and columns equal to number of columns of 2nd matrix (B)