Its called the Difference of Perfect Squares:
5 is the square root of 25
x is the square root of x^2
(-8)(+8) is the square root of -64
You can FOIL the factored answer to check your answer:
First: 5x times 5x = 25x^2
Outside: (-8 times 5x) = -40x
Inside: (+8 times 5x) = +40x
+40x + -40x = 0x so cancel each other out
Last= -8 times +8 = -64
So the unfactored expression
F= 25x^2
OI= 0
L= -64
25x^2-64