Explanation:
Heron's formula when all 3 sides (a, b, c) are given :
s = (a + b + c)/2
A = sqrt(s(s-a)(s-b)(s-c))
in our case
s = (34+73+89)/2 = 98
A = sqrt(98(98-34)(98-73)(98-89)) =
= sqrt(98×64×25×9) = sqrt(98)×8×5×3 =
= sqrt(2×49)×120 = sqrt(2)×7×120 =
= sqrt(2)×840 = 1,187.939392... ≈ 1,188 in²