x² - 2.2x + c

perfect square trinomials can be factored into identical binomials, 

(x + a)² = (x + a)(x + a) = x² + 2ax + a²

Compare the general form to your expression

x² + 2ax + a²
x² - 2.2x + c

We need to find the value of a and square it to replace c = a²

if 2a = -2.2

a = -1.1, then a² = (-1.1)² = 1.21

x² - 2.2x + 1.21 = perfect square trinomial, which factors into (x + a)², and we found a = -1.1

(x - 1.1)²