Don't be put off by the power 23. Start by thinking about the matrix . If A is invertible then so is , which means that cannot send the nonzero vector to the zero vector. Therefore A is not invertible, which means that its determinant must be 0. If you write down the determinant of A and put it equal to 0 then you get a quadratic in a, which has two complex solutions. I guess those two values are what the question is asking for.