$$5(-3p-4)=-15p-20$$
$p\in \mathrm{C}$
$$-15p-20=-15p-20$$
$$-15p-20+15p=-20$$
$$-20=-20$$
$$\text{true}$$
$$p\in \mathrm{C}$$
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$p\in \mathrm{R}$
$$p\in \mathrm{R}$$