[tex]a^4+3a^2-(5a-3)(-7a)\ \ \ \ |\text{use distributive property}\\\\=a^4+3a^2-[(5a)(-7a)+(-3)(-7a)]\\\\=a^4+3a^2-(-35a^2+21a)\\\\=a^4+3a^2+35a^2-21a\\\\=a^4+38a^2-21a[/tex]
