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    The determinant of the left hand side of the system reduces to D = 2a² - 2a - 4. For the system to have no solution, D = 0, solving the quadratic equation yields:

    a² - a - 2 = 0;

    (a + 1)(a - 2) = 0;

    a = -1, a = 2

    The least possible value of a is -1.