Could an answer to factorise x 2 -10x+25 be x(x-10)+25?

I don't think so, the question looks like a parabola. You're not factorizing everything, maybe you're thinking of group in pairs. 

For the answer, c(25) square rooted is 5 so you're looking at the perfect squares. 

(x-5)²

or (x-5)(x-5)

Hope that helped