Question 295448
You didn't specify whether you wanted the <b>total</b> surface area (including the area of the base) or just the <b>lateral</b> surface area, the sides only.

For the <b>lateral</b> surface area of a right cone the formula is {{{ LSA = pi*r*h }}}, where h is the height of the cone. <br>
For the <b>total</b> surface area of the same cone the formula is {{{ TSA = pi*r*h + pi(r)^2 }}}, where h is the height of the cone.<hr>
Substituting your numbers of r = 9 and h = 12, we get
{{{ LSA = pi*9*12 = 108*pi }}} = approximately 339.29 in^2
{{{ TSA = pi*9*12 + pi(9)^2 = 108*pi + 16 }}} = approximately 355.29 in^2 & bob's your uncle!