Consider the following expressions:
λx.λy.λz.(x λz).λy.(x y) (λx.λy.(x λz.y)) (λx.λy.λz.x λz.(λy.(x y))) (λx.(x x) λy.(x y x))
How many of these expressions are λ expressions?
If you cannot work it out in your head, use pencil and paper to build parse trees.
Review the three productions in the lambda calculus grammar and study until you know them cold.
Pay special attention to the constraints on the location of parentheses.