In the proportion 7 : 12 :: 21 : 36, what is the product of the means?

The means are the two middle terms, 12 and 21, so their product is $12\times 21=252$. The extremes 7 and 36 give $7\times 36=252$ as well, which is exactly why this is a proportion. 147 is $7\times 21$ and 432 is $12\times 36$ — those pair up the wrong terms, and 76 is the sum of all four.