10" pizza sells for $2.99 => 2.99/10^2= $(299/10000) per unit area.

12" pizza, priced at $ 3.66 => 3.66/ 12^2 = $(366/14400) per unit area.

the discount is given by

(299/10000)-(366/14400)/(366/14400) = (269/1794) * 100%

the answer is not exactly 15%