Rewrite $ m^2-64n^2 $ as:
$$ \color{blue}{ m^2-64n^2 = (m)^2 - (8n)^2 } $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = m $ and $ II = 8n $ , we have:
$$ m^2-64n^2 = (m)^2 - (8n)^2 = ( m-8n ) ( m+8n ) $$