Let's simplify the given expression step by step:

The expression is:

$6 - 2y + (-8y) + 4.2$

1. Combine the constants: $6 + 4.2 = 10.2$
2. Combine the like terms for $y$: $-2y - 8y = -10y$

Now, we have:

$10.2 - 10y$

This is the simplified expression. If you want to solve it further, you would need a value for $y$.

Let me know if you have any other questions or need more details!

Here are some related questions for you:

1. How do you factor expressions like $10.2 - 10y$?
2. What if we set the expression equal to zero? How would we solve for $y$?
3. What is the significance of combining like terms in algebra?
4. How do you handle negative coefficients when simplifying expressions?
5. How can decimals like 4.2 be simplified in algebraic expressions?

Tip: Always combine like terms first to make simplifying expressions easier!