Did you learn how to factor the difference of two squares ?
This thing is the difference of two squares.
The first term is the square of 5x .
The second term is the square of 4 .
When it's factored, it's
(sum of the square roots) x (difference of the square roots) .
25 x² - 16 = (5x + 4) times (5x - 4) .
To check it, go ahead and FOIL that out, and you'll get 25x²-16 .