21: D( f ) = ℝ; f is continuous there.
y-intercept: f (0) = −1; x-intercepts: f = 0 gives x = −1, x = 1.
f (−x) = f (x), hence f is even.
Limits at endpoints:

Interpretation as asymptotes:
No horizontal asymptote at −∞, but a chance for oblique.

So no oblique there.
No horizontal asymptote at ∞, but a chance for oblique.

So no oblique there (this also follows by symmetry).

Now determine monotonicity using f ′.

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Answer