Remove the parentheses, rewrite subtraction as addition of the opposite, reorder, and combine like terms.

$\displaystyle{15}{x}{y}+{\left(-{6}{y}{z}\right)}+{9}{z}+{3}{x}{y}+{6}{y}{z}+{\left(-{9}{z}\right)}={15}{x}{y}+{3}{x}{y}+{\left(-{6}{y}{z}\right)}+{6}{y}{z}+{9}{z}+{\left(-{9}{z}\right)}\usepackage{cmbright}\Huge\$ $\displaystyle={\left({15}+{3}\right)}{x}{y}+{\left(-{6}+{6}\right)}{y}{z}+{\left({9}+-{9}\right)}{z}={18}{x}{y}+{0}{y}{z}+{0}{z}={18}{x}{y}\usepackage{cmbright}\Huge\$