Find all the second partial derivatives. f(x, y) = x^8y^9 - 9x^6y.
a) f_{xx} = 4032x^6y^9 - 3240x^4y
b) f_{yy} = 72x^8y^7 - 54x^6
c) f_{xy} = 72x^7y^8 - 54x^5
d) f_{yx} = 4032x^5y^{10} - 3240x^3