To combine like terms in the expression $3a + 5 - x + 7x - 2a$, follow these steps:

1. Identify like terms:

   • The terms involving $a$ are $3a$ and $-2a$.
   • The terms involving $x$ are $-x$ and $7x$.
   • The constant term is $5$.

2. Combine the like terms:

   • For $a$: $3a - 2a = 1a = a$
   • For $x$: $-x + 7x = 6x$
   • The constant term remains $5$.

3. Write the simplified expression: $a + 6x + 5$

So, the simplified expression is $a + 6x + 5$.

Would you like more details on any part of this process? Here are five related questions you might find interesting:

1. How do you combine like terms in an equation involving multiple variables?
2. What are the rules for combining like terms with different exponents?
3. Can you explain the difference between like and unlike terms?
4. How does distributing affect the combination of like terms?
5. What is the significance of combining like terms in solving equations?

Tip: When combining like terms, always pay close attention to the signs (+ or -) in front of each term.