$=2m^{2}-9m+9-(-2m^{2}-7m-9)$
Apply the distributive law $-\left(-a-b\right)=a+b$$=2m^{2}-9m+9+2m^{2}+7m+9$
Group like terms$=2m^{2}+2m^{2}-9m+7m+9+9$
Add similar elements: $ 2m^{2}+2m^{2}=4m^{2}$$=4m^{2}-9m+7m+9+9$
Add similar elements: $ -9m+7m=-2m$$=4m^{2}-2m+9+9$
Add the numbers: $ 9+9=18$$=4m^{2}-2m+18$