I think this wants you to use the rule 1/2absinC = A, in words "the area of a triangle = 1/2 the product of two sides multipied be the sine of the angle between them so in you case 1/2 x 12 x 5sinC = 18

30sinC = 18

sinC = 3/5

C = 36.86989765 degrees

Hope that helps!