Step 1: Concept
[tex]\begin{gathered} \text{from} \\ x\text{ = - 3 and x = 7} \\ (x+\text{ 3) (x - 7) = 0} \\ x^2\text{ - 7x + 3x - 21 = 0} \end{gathered}[/tex][tex]x^2-4x\text{ - 21 = 0}[/tex]Since it has a maximum
You multiply by negative
Then we have
[tex]-x^2\text{ + 4x + 21 = 0}[/tex]