$\displaystyle{{\left({7}{{x}}^{{3}}-{9}{{x}}^{{2}}\right)}}^{{2}}={49}{{x}}^{{6}}-{126}{{x}}^{{5}}\usepackage{cmbright}\Huge\$ ___

Fill in the blank with the missing term, including the proper symbol: + or –. Use the symbol ^ to represent exponents. For example, write the exponent $\displaystyle{{a}}^{{b}}\usepackage{cmbright}\Huge\$ as a^b.