Extra explanation: graph

Introduction: A function of the form $y(x)=ax^2+bx+c$, where $a$, $b$ and $c$ are numbers ($a\neq 0$) is called a quadratic function.

Graph: The graph of a quadratic function is a parabola; if $a>0$ the parabola is opened upward and if $a<0$ opened downward.

Hence, a quadratic function $y(x)=ax^2+bx+c$ with $a>0$ looks as follows.



And a quadratic function $y(x)=ax^2+bx+c$ with $a<0$ looks as follows.