Cześć!
1)
[tex](7+9x)+(5x-4)=7+9x+5x-4=\\\\=9x+5x+7-4=\boxed{14x+3}[/tex]
2)
[tex](y+7)-(y^2+y-5)=y+7-y^2-y+5=\\\\=-y^2+y-y+7+5=\boxed{-y^2+12}[/tex]
3)
[tex](-2a^2-3ab+c)-(-2a^2+2ab+5c)=\\\\=-2a^2-3ab+c+2a^2-2ab-5c=\\\\=-2a^2+2a^2-3ab-2ab+c-5c=\boxed{-5ab-4c}[/tex]