Use the law of cosines or Heron's formula.

Let us use law of cosines, because you have a better chance of hearing of it.

#1] Let a=3 b=4 and c=6.

Then, we can find all the angles of this triangle.

Say angle A.

a^2=b^2+c^2-2bc*cos(A)

9=16+36-48*cos(A)

You can solve for cos(A) and then take arccos to get the angle.

Now do that will all angles.

Then you can determine whether the triangle if acute or not.