Let \( a, b, c \) be the three vectors such that \( c \neq 0 \) and
\[
a \cdot b = 2a \cdot c, \quad |c| = |c| = 1, \quad |b| = 4, \quad |b \times c| = \sqrt{15}, \quad \text{if} \quad b - 2c = \lambda \alpha, \quad \text{then} \quad \lambda \quad \text{equals}
\]