Lets hope this is right after having not touched first year maths for so long haha
\text{The dot product is defined as } \mathbf{a}\cdot \mathbf{b} = \|\mathbf{a}\| \|\mathbf{b}\| \cos\theta
\text{Applying this,}
LHS = (0,1,-1) \cdot (2, 1, M) = 1 - M
\begin{align*} RHS &= \|(0,1,-1)\| \|...