What is the value of x2-4x+4

Unclear. What do you mean by "value of?" Perhaps you meant, "What are the factors of x^2-4x+4?" Look at the final 4. Possible factors of 4 are (-1) (-4) and (-2) (-2). Note that - 2-2 = - 4, which matches the "-4" in x^2-4x+4.

Thus, try the following factors: (x-2) (x-2). When mult. together, x^2-4x+4 results, so these factors are the correct ones.