Factor completely x2 − 10x + 25.
(x − 5)(x − 5)
(x + 5)(x + 5)
(x + 5)(x − 5)
(x − 25)(x − 1)

The answer is:
(x - 5) (x - 5)   The equation is a perfect square. 
(a - b) (a - b)   The "formula" goes a^2 - 2ab + b^2 for this equation so when you expand out the brackets x to x, x to 5 then 5 to x and 5 to 5 ( a x a, a x b, b x a, b x b) then collect all the like terms it turns into the equation above. 
I hope that made some sense ^ ^" and it helped in some way.