Answer
$K = 56~eV$
Work Step by Step
According to the diagram, an electron in the $n=2$ state has an energy of $106~eV$. If an energy of $400~eV$ is absorbed, then the electron can have a total energy of $506~eV$
The electron needs an energy of $450~eV$ to escape from the well.
We can find the remaining energy:
$K = 506~eV-450~eV = 56~eV$
