Answer:
(7 + 2√u)(7 - 2√u)
Explanation:
There are two terms here, 49 and -4u. These terms have nothing in common, so any factors will not be rational unless u is a positive perfect square.
However, we could rewrite 49 - 4u as 7^2 - [2√u]^2, which is the difference of two squares. This latter result can be factored as
(7 + 2√u)(7 - 2√u)