A) Show that x² - 8x + 20 can be written in the form (x - a)² + a where a is an integer. b) Hence explain how you know that x² - 8x + 20 is always positive.

I have worked out the first bit of the question and the answer is:
x²-8x+20
x²-8x+16-16+20
(x-4)²+4
(x-a)²+a
a= 4

I just need help with part b and I'm struggling to do it. Please help.