Let's simplify the expression step by step:

Given:
$2x + 3x + 6 - 12$

1. Combine like terms (the terms involving $x$): $2x + 3x = 5x$

2. Now, handle the constants (the numbers without $x$): $6 - 12 = -6$

3. So the simplified expression is: $5x - 6$

This is the final simplified form of the expression.

Would you like more details or have any questions about this process? Here are 5 related questions to explore further:

1. How do you solve an equation if the expression is equal to something, like $5x - 6 = 0$?
2. How do you factor expressions like $5x - 6$?
3. What if the terms have higher powers of $x$, such as $x^2$?
4. How do you solve for $x$ when there are fractions in the equation?
5. What happens if we add or subtract more terms with $x$ in the original expression?

Tip: When simplifying expressions, always combine like terms (those with the same variable and exponent) first!