Dimitri determined that the ordered pair (2, –2) is a solution to the system of linear equations 7x + 9y = –4 and 5x – 2y = 6 as shown. Equation 1: 7 x + 9 y = negative 4. 7 (2) + 9 (negative 2) = negative 4. 14 + (negative 18) = negative 4. Negative 4 = negative 4. Equation 2: 5 x minus 2 y = 6. 5 (2) minus 2 (negative 2) = 6. 10 minus 4 = 6. 6 = 6. What was Dimitri’s mistake? He mixed up the coordinates of the ordered pair when substituting it into the equations 7x + 9y = –4 and 5x – 2y = 6. He checked the equation 7x + 9y = –4 first when he should have checked 5 x minus 2 y = 6 first. He made a mistake in his calculations when substituting the ordered pair into the equation 7x + 9y = –4 and simplifying. He made a mistake in his calculations when substituting the ordered pair into the equation 5x – 2y = 6 and simplifying.
