He mixed up the coordinates of the ordered pair when substituting it into the equations 7 x plus 9 y is equal to negative 4 and 5 x minus 2 y is equal to 6 .
B
He checked the equation 7 x plus 9 y is equal to negative 4 first when he should have checked 5 x minus 2 y is equal to 6 first.
C
He made a mistake in his calculations when substituting the ordered pair into the equation 5 x minus 2 y is equal to 6 and simplifying.
D
He made a mistake in his calculations when substituting the ordered pair into the equation 7 x plus 9 y is equal to negative 4 and simplifying.