$$3 ( x y + z ) + 0 = 3 ( x y + z )$$
$x\in \mathrm{R}$
$$3xy+3z+0=3\left(xy+z\right)$$
$$3xy+3z=3\left(xy+z\right)$$
$$3xy+3z=3xy+3z$$
$$3xy+3z-3xy=3z$$
$$3z=3z$$
$$z=z$$
$$\text{true}$$
$$x\in \mathrm{R}$$
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$y\in \mathrm{R}$
$$y\in \mathrm{R}$$
