I’m assuming it’s a right triangle and you’re finding the hypotenuse. C is always the hypotenuse.
a^2+b^2=c^2
8^2+7^2=c^2
64+49=c^2
113=c^2
c=sqrt113
If it’s another side just sub it in.
8 is largest so it’s c.
using the same formula
64=49+b^2
64-49=b^2
15=b^2
b=sqrt15
