Prove that L = { wu; wu in {a,b}* and |w| = |u| } is not regular
As Rick Decker said $L$ is a regular language. For example:
$$L= \mathscr{L}(\,(aa | ab | ba | bb)^*\,)$$ 

If your teacher asked you to prove that $L$ is not regular in an exam then he is a big troll. Otherwise remember that the pumping lemma is only usefull to prove that a language is not regular and if you fail to find the contradiction in the lemma then that doesn't prove anything. $L$ can be regular or irregular at that point.

In this answer I covered some common mistakes when students use the pumping lemma: Common Mistakes