Find dxy using the rules for finding derivatives.
=(5x^2−8)(9x^2−3x+5)

You can do foil method first, to make it easier and to avoid product rule,
y= 45x^4 - 15x³ + 25x² - 72x² + 24x - 40
Simplify like terms,
y= 45x^4 - 15x³ - 47x² + 24x - 40
Now, find derivative,
dy/dx  = 180x³ - 45x² - 94x + 24