The expression $-6 \equiv 4 \pmod{10}$ means that when $-6$ is adjusted to fall within the standard range for units digits (0 to 9), it becomes 4. This is a concept from modular arithmetic.

Here’s how it works:

1. Starting with $-6$: The number $-6$ is not in the standard range for units digits, which is from 0 to 9.

2. Adding 10 (the modulus): To bring $-6$ into this range, you add 10 (because we are working modulo 10): $-6 + 10 = 4$ So, $-6$ is equivalent to $4$ when considering numbers modulo 10.

In modular arithmetic, numbers are considered equivalent if they differ by a multiple of the modulus (in this case, 10). Since $-6$ and 4 differ by 10, they are equivalent under modulo 10.

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

1. What is modular arithmetic, and how does it work?
2. How can you solve modular arithmetic problems involving negative numbers?
3. What are the applications of modular arithmetic in cryptography?
4. How does modular arithmetic help in finding the remainder?
5. Can modular arithmetic be extended to other moduli, like 12 or 7?

Tip: When dealing with negative numbers in modular arithmetic, remember to add the modulus until you get a positive equivalent!