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If it is not the case that the zoo contains both bears and yaks, then it contains lions and zebras.

Is this the right diagram?

~B & ~Y -> L & Z

~L or ~Z -> B or Y

I know this looks like the SuperPrep B rule, but in this case I am asking about this rule as it stands.

Not quite. If there are either bears and no yaks or yaks and no bears, then lions and tigers still must be present. It should be written as this:

~(B and Y) -> L and Z

This is equivalent to :
~B or ~Y -> L and Z
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The contrapositive is:
~(L and Z) -> ~~ (B and Y)

This is equivalent to:
~L or ~Z -> B and Y

Hope this helps!

We just did a whole long thread about this rule like a week ago, coincidentally.