Given the 2 by 2 matrix, row 1, column 1, 5 column 2, negative 4 row 2, column 1, 3 column 2, 6 the 2 by 1 column matrix x y is equal to the 2 by 1 column matrix 12 66 , what is the 1 by 1 row matrix column 1 cap A sub x ?

To form A_x, replace the first column of the coefficient matrix with the constants on the right-hand side, 12 and 66. Keep the second column, −4 and 6, unchanged. This gives the matrix with rows (12, −4) and (66, 6), which is choice C.

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