One of the tools that's great to keep in your algebra toolbox for the rest of your life is:
The product of (sum of 2 things) (difference of 2 things) = (difference in their squares).
For example: (x + y) (x - y) = (x² - y²)
When you're trying to factor algebra expressions, it's always good to keep your
eyes open for the difference of two squares, and remember that it's going to be
(sum) x (difference) of the things that they're squares of.
Here you have (a sum) x (a difference) of the same 2 things:
(5a + 2bc) (5a - 2bc)
You can multiply them out, or just use the little tool above:
(5a)² = 25a²
(2bc)² = 4b²c²
So (their sum) x (their difference) is (25a² - 4b²c²) .
