Answer
the terms are unlike terms/radicals, and hence cannot be combined
Work Step by Step
The result of $28-4\sqrt{2}=24\sqrt{2}$ is incorrect because the terms (or radicals) are unlike. This can easily be seen by expressing the given as
\begin{align*}
&
28\sqrt{1}-4\sqrt{2}
.\end{align*}
Since the radicals are unlike (i.e. the radicands are different), then the expression cannot be combined to become $24\sqrt{2}$.
