Answer
$(x-2)^2.$
Work Step by Step
A perfect square trinomial is factoerd by reversing the following formulas:
The square of a binomial sum is: $(A+B)^2=A^2+2AB+B^2$.
The square of a binomial difference is: $(A-B)^2=A^2-2AB+B^2$.
E.g. $x^2-4x+4=x^2-2\cdot x\cdot2+2^2=(x-2)^2.$
