Hi, I'm looking for a bit of help on question 4biii. I understand what has been done, but not why. You've factorised x^3-4x+15=0 earlier in the question, so the mark scheme just shows that the discrimant of the quadratic factor is less than 0 so has no real roots, making the only real root -3. What I don't understand is how the fact that -3 is the only real root means that it's the only stationary point. I was only told that a root is when the curve crosses the x axis!
Could anyone be awesome enough to help me out?