Step-by-step explanation:
if I understand you correctly, then the 3 sides are
a = 20 m
b = 34 m
c = 42 m
under this assumption the best is to use Heron's Formula to get the area :
s = (a + b + c)/2
area = sqrt(s(s - a)(s - b)(s - c))
in our case
s = (20 + 34 + 42)/2 = 96/2 = 48
area = sqrt(48(48-20)(48-34)(48-42)) =
= sqrt(48×28×14×6) = sqrt(112,896) =
= 336 m²
