25x^2-16 how to factor

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 .