a , a+2 , a+4
O = a + a+2 + a + 4 = 3a + 6
5/12 = (a+4)/(3a+6)
5(3a+6) = 12(a+4)
15a + 30 = 12a + 48
3a = 18
a = 6
a + 2 = 6+2 = 8
a + 4 = 6 + 4 = 10
O = 6 + 8 + 10 = 24
p = O/2 = 24/2 = 12
P = √p(p-a)(p-b)(p-c) = √12(12-6)(12-8)(12-10) = √12*6*4*2 = √576 = 24 [j²]