I had did it for a few minutes ago :v 

a) (x - 3)³ - (x - 3)(x² + 3x + 9) + 6(x + 1)² = 15

(x - 3)*[(x - 3)² - (x² + 3x + 9)] + 6(x² + 2x + 1) = 15

(x - 3)*[x² - 6x + 9 - x² - 3x - 9] + 6x² + 12x + 6 = 15

(x - 3)*(-9x) + 6x² + 12x + 6 = 15

-9x² + 27x + 6x² + 12x + 6 = 15

39x - 3x² = 9

13x - x² = 3

x² - 13x = -3

x² - 13x + 3 = 0

\(x^2-2.x.\dfrac{13}{2}+\dfrac{169}{4}-\dfrac{157}{4}=0\)

\(\left(x-\dfrac{13}{2}\right)^2=\dfrac{157}{4}\)

\(\Leftrightarrow\left[{}\begin{matrix}x-\dfrac{13}{2}=\dfrac{\sqrt{157}}{2}\\x-\dfrac{13}{2}=\dfrac{-\sqrt{157}}{2}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{13+\sqrt{157}}{2}\\x=\dfrac{13-\sqrt{157}}{2}\end{matrix}\right.\)

b) x(x - 5)(x + 5) - (x + 2)(x² - 2x + 4) = 3

x(x² - 25) - (x³ + 8) = 3

x³ - 25x - x³ - 8 = 3

-25x = 11

\(x=-\dfrac{11}{25}\)