Answer
$y=x^2+4x-21$
Work Step by Step
Recall:
If $a$ and $b$ are zeros of a quadratic function and there is no constant multiplier, then the equation of the function is $y=(x-a)(x-b)$.
The quadratic function has the zeros $3$ and $-7$ and there is no constant multiplier.
Thus, the equation of the quadratic function must be:
\begin{align*}
y&=(x-3)[x-(-7)]\\
y&=(x-3)(x+7)\\
y&=x^2+7x-3x-21\\
y&=x^2+4x-21
\end{align*}