What must be true for an expression to be a difference of two squares

I think it must have a power like squared or cubed also I think it must have a subtraction sign...
Some examples of difference of two squares:
9x^2 - 1
Which would be = (3x+1) (3x-1)
4n^2 - 49
Which would be = (2n-7) (2n+7)
249r^2 - 150
Which would be = 6(7r+5) (7r-5)
150k^2 - 216
Which would be = 6(5k+6) (5k-6)
Notice that each bracket for each answer, one bracket has a + sign and another has a - sign.

Not sure what the question is but i hope this x = years100 * x