I’m assuming the numbers you have given are the lengths of the sides. Then you use Heron’s formula. First u work out a given by equation s=(a+b+c)/2 then the area is given by the equation area=square root of (s(s-a)(a-b)(a-c)). So for your question s=(17+8+15)/2=40 and area=square root of (40x(40-17)x(40-8)x(40-15))= sqrt(40x23x32x25)=857.9 to 1dp
It depends on the triangle, and there is a whole topic that you need to know that I could teach on just triangles.