I don’t understand this,can you explain it simply

Question

What is the condition that the cubic $y=ax^3+bx^2+cx+d$ shall have two extremes?

Answer 1

HINT

Recall that for a differentiable function the extrema points can be found among the "critical points" which satisfy the condition $$f'(x)=0$$ and that for a cubic this leads to a quadratic equation.

What is, in that case, the nature of those critical points if they are two distinct points?