This has the same rules as double brackets, except you have to multiply 3 terms by 2 terms. First you multiply them all by 5x^2:
3x^2 × 5x^2 = 15x^4
6x × 5x^2 = 30x^3
-6 × 5x^2 = -30x^2
Now by -3:
3x^2 × -3 = -9x^2
6x × -3 = -18x
-6 × -3 = 18
So then you add up the like terms and you get 15x^4 + 30x^3 - 39x^2 - 18x + 18, which is as far as you can simplify. I hope this helps!
