A ᑌ (B ᑎ C)

Substitute the sets for the letters:


{9, 10, 11, 12} ᑌ ({9, 10, 13, 14} ᑎ {11, 13, 15})

Do between the parentheses first.

"ᑎ" says "Make a set that contains only
the elements which the two sets on my
right and my left have in common.

So the two sets that "ᑎ" has on each side
of it are these:

{9, 10, 13, 14} and {11, 13, 15}

They only have one element in common, 13, so we put
13 in a set like this {13}, and replace
what is in the parentheses with it.  So now we have
only this:

{9, 10, 11, 12} ᑌ ({13})

and so we can drop the parentheses and just have

{9, 10, 11, 12} ᑌ {13}

"ᑌ" says "Make a set that contains ALL
the elements which the two sets on my right and 
my left contain, whether they are in common or not.

So we put them all together in a set:

{9, 10, 11, 12, 13}

That's the answer.

Edwin