9a² – m² + 10mn – 25n²

Write the last three terms in parentheses
preceded by a minus sign:

9a² – (m² - 10mn + 25n²)

9a² - (m-5n)²

That's the difference of two squares:

[3a - (m-5n)][3a + (m-5n)]

[3a - m + 5n][3a + m - 5n]

(3a - m + 5n)(3a + m - 5n)

Edwin