In this case, we have to remember the meaning of the nomenclature "18:2Δ9,12". Where 18 is the <u>number of carbon atom</u>s, 2 is the <u>number of double bonds,</u> and the numbers successive to Δ "delta" the position of the double bonds <u>starting</u> to count from the carboxylic -COOH end of the molecule.
In other words, the main functional group is a <u>carboxylic acid</u>. We have a total of 18 carbons. Additionally, we have 2 double bonds. On carbons 9 and 12.