The given expression is $25x^2 - 81$, and you're asked to factor it using the difference of squares method.

The difference of squares formula is:

$a^2 - b^2 = (a - b)(a + b)$

In this case:

• $25x^2$ is the square of $5x$, because $(5x)^2 = 25x^2$,
• $81$ is the square of $9$, because $9^2 = 81$.

So, applying the difference of squares formula:

$25x^2 - 81 = (5x - 9)(5x + 9)$

The factored form of $25x^2 - 81$ is:

$(5x - 9)(5x + 9)$

Would you like further details or have any questions about this?

Here are 5 related questions for further practice:

1. How do you apply the difference of squares formula in general?
2. Can all quadratic expressions be factored using the difference of squares?
3. What happens if the terms aren't perfect squares? How can you handle that?
4. How do you recognize the difference of squares when variables are involved?
5. What are some real-world applications of factoring quadratic expressions?

Tip: Always check if both terms in a quadratic expression are perfect squares when using the difference of squares method.