Use the special cases of squared polynomials:
[tex] (a+b)^2 = a^2 + 2ab + b^2 [/tex]
Substitute the given equation.
[tex] (a-2)^2 = a^2 + 2(a)(-2) + (-2)^2 [/tex]
[tex] a^2 -4a+4 [/tex]
