The main concept being tested here is what the discriminant values of a quadratic equation represent should they be positive, negative or zero. The fact that the line is a tangent to the curve indicates that the quadratic equation derived from subtituting the equation of the line into the equation of the curve means that it has two repeated real roots since the curve and the line only meet at one point, which is the tangential point/point of contact. Hopefully this has given you a better understanding of discriminants and their properties. Cheers and all the best! :)