Answer:
A = 88.18
Solution:
s = a + b + c / 2
s = 12 + 15 + 21 / 2
s = 24
A = s(s-a)(s-b)(s-c)
A = √24×(24 - 12)×(24 - 15)×(24 - 21)
A = 88.18 ← Final answer