Since you know the lengths of the sides, you can use Heron's formula:
A = sqrt[ s · (s - a) · (s - b) · (s - c) ]
where a, b, and c are the sides
and s is the semiperimeter: s = (a + b + c) / 2.
First: find the semiperimeter: a = (8 + 5 + 7) / 2 = 10
Then: find the area: A = sqrt[ 10 · (10 - 8) · (10 - 5) · (10 - 7) ] = sqrt[ 300 ]