Determine All Values for a Such That the Vectors $\mathbf { x } = 2 \mathbf { i } + \mathbf { j } + 2 \mathbf { k }$

Essay

Determine all values for a such that the vectors $\mathbf { x } = 2 \mathbf { i } + \mathbf { j } + 2 \mathbf { k }$ and $\mathbf { y } = \mathbf { i } + 2 \mathbf { j } + a \mathbf { k }$ will form a 60° angle.