If $f: \Re $ is defined by $f(x)= \left\{ \begin{array}{1 1} \frac{\cos 3x- \cos x}{x^2} & \quad for\;x\neq 0 \\ \lambda & \quad for\;x=0 \end{array}. \right. $ and if f is continuous at $x=0,$ then $\lambda=$

Question

\[\begin {array} {1 1} (1)\;-2 & \quad (2)\;-4 \\ (3)\;-6 & \quad (4)\;-8 \end {array}\]