Simplify the expression.

2y³ · 4y³  =  (2 · 4) · (y³ · y³)  =  8y⁶         (multiply 2 x 4 and add the exponents of the y-terms)

-x⁴ · 3y^-3 · 5y⁶ · 6x^-9  =  (-1 · 3 · 5 · 6) · (x⁴ · x^-9) · (y^-3 · y⁶)  =  -90x^-5y³   (again, multiply the numbers and add the exponents of the like terms)