(3+6z+x)(-2)

Just multiply each term by -2.

Take this example:

\begin{array}{rcl}(-3)(2 + 5x - 8y) &=& (-3 \times2) + (-3 \times 5x) + (-3 \times -8y) \\ &=& -6 + (-15x) + (+24y) \\ &=& -6 - 15x + 24y\end{array}

Advanced?