I'm stuck at the properties of a 3d curve parametrized by arc length. My problem is given below.

We know that the binormal of a curve at a point is:

where binormal and is normal to the curve and is tangent.

So binormal is the cross product of normal and tangent at a point of a curve.

Now if why's that ? How do one prove this statement?

What properties of mathematics verify that ? Is it possible to kindly help me answer this question?