a) `-8ab^3(4a^3b - 1/4a^2 + 1/5 ab)`
`= -8ab^3 . 4a^3b + 8ab^3 . 1/4a^2 - 8ab^3 . 1/5ab`
`= (-8.4)(a.a^3)(b^3.b) + (8. 1/4)(a.a^2)b^3 - (8 . 1/5)(a.a)(b^3.b)`
`= 2a^3b^3-32a^4b^4 - 8/5a^2b^4`
b) `(x^2+4x^2)^2 + 9x^2 - 6x(x^2+4x)`
`= x^4 + 8x^4 + 16x^4 + 9x^2 - 6x^3 - 24x^2`
`= (x^4 + 8x^4 + 16x^4) - 6x^3 + (9x^2 - 24x^2)`
`= 25x^4 - 6x^3 - 15x^2`