Let's simplify the expression $7 - 9X - 15 + 12X$.

1. Combine the like terms (terms with $X$ and constant terms):

$7 - 15 + (-9X + 12X)$

2. Simplify the constants and the coefficients of $X$:

$(7 - 15) + (-9X + 12X) = -8 + 3X$

The simplified expression is:

$3X - 8$

Would you like any more details or have any other questions? Here are some related questions you might consider:

1. How do you combine like terms in an expression?
2. What is the distributive property and how is it used in algebra?
3. How can you factor the expression $3X - 8$?
4. What are the rules for adding and subtracting polynomials?
5. How does the order of operations affect the simplification of expressions?

Tip: Always simplify expressions by first combining like terms, then arrange the terms in a standard form.