The simplified form of the expression:

$(-x^2 - 5x) - (-6x^2 + 5x + 6)$

is:

$5x^2 - 10x - 6$

Would you like further details or have any questions?

Here are five related questions you might find useful:

1. How do you expand algebraic expressions involving multiple parentheses?
2. How do you combine like terms in a polynomial?
3. What are the steps to factor a quadratic expression like $5x^2 - 10x - 6$?
4. How do you solve quadratic equations using the quadratic formula?
5. How do you graph quadratic functions?

Tip: Always distribute negative signs carefully when simplifying algebraic expressions.