#### Answer

$\color{blue}{\left\{-7, 4\right\}}$

#### Work Step by Step

RECALL:
A trinomial of the form $x^2+bx+c$ can be factored if there are integers $d$ and $e$ such that $c=de$ and $b=d+e$.
The trinomial's factored form will be:
$x^2+bx+c=(x+d)(x+e)$
The trinomial in the given equation has $b=3$ and $c=-28$.
Note that $-28=7(-4)$ and $3= 7+(-4)$.
This means that $d=7$ and $e=-4$
Thus, the factored form of the trinomial is: $(p+7)[p+(-4)]=(p+7)(p-4)$
The given equation maybe written as:
$(p+7)(p-4)=0$
Use the Zero-Factor Property by equating each factor to zero.
Then, solve each equation to obtain:
\begin{array}{ccc}
&p+7 = 0 &\text{ or } &p-4=0
\\&p=-7 &\text{ or } &p=4
\end{array}
Thus, the solution set is $\color{blue}{\left\{-7, 4\right\}}$.