Please Help with this question?

How would you find the area of a triangle with sides 3,5,7. i tried finding the height but failed epically since it does not give you the angles. i also used hero's formula but that didn't work either cause i got a non-terminating number(think they're called irrational numbers). anyways i need to find the area of the triangle so i can find the area of a triangular prism with the height of 10. but if you could explain how to find the area of the triangle in radical form it would be greatly appreciated.