Hello,
So if we have the equation: -7y^3(4y + y - 3), we're essentially distributing "-7y^3" to everything of what's in the parenthesis. When you distribute -7y^3, know that you add exponents but you multiply the numbers. After simplifying you get:
-28y^4 - 7y^4 + 21y^3
**We can still simplify this expression because there are still like terms:
-35y^4 +21y^3
Hope this helps!
May