Step 1 :
After factoring out $ 4 $ we have:
$$ 64a^{4}-224a^{2}+196 = 4 ( 16a^{4}-56a^{2}+49 ) $$Step 2 :
Both the first and third terms are perfect squares.
$$ 16x^4 = \left( \color{blue}{ 4a^{2} } \right)^2 ~~ \text{and} ~~ 49 = \left( \color{red}{ 7 } \right)^2 $$The middle term ( $ -56x^2 $ ) is two times the product of the terms that are squared.
$$ -56x^2 = - 2 \cdot \color{blue}{4a^{2}} \cdot \color{red}{7} $$We can conclude that the polynomial $ 16a^{4}-56a^{2}+49 $ is a perfect square trinomial, so we will use the formula below.
$$ A^2 - 2AB + B^2 = (A - B)^2 $$In this example we have $ \color{blue}{ A = 4a^{2} } $ and $ \color{red}{ B = 7 } $ so,
$$ 16a^{4}-56a^{2}+49 = ( \color{blue}{ 4a^{2} } - \color{red}{ 7 } )^2 $$