The third λ-calculus problem is a bit more advanced (although still simple):

Reduce the following expression, using the beta-rule, to no more than one term. If the expression cannot be reduced, enter "CAN'T REDUCE".
((λx.(x x)) (λx.(x x)))

"beta-reduction" is just recution by substituting variable value in the function body with its value. Check Reductions #1 and Reductions #2 for more examples.

The expression ((λx.(x x)) (λx.(x x))) cannot be reduced. It is easy to see that applying reduction results with exactly the same expression. So the answer is "CAN'T REDUCE".