The concentration of an organic compound in chloroform is $6.15$g per $100\;mL$ of solution. A portion of this solution in a $5\;cm$ polarimeter tube causes an observed rotation of $-1.2^{\circ}.$ What is the specific rotation of the compound ?

Question

$$\begin {array} {1 1} (1)\;+ 12 & \quad (2)\;-3.9^{\circ} \\ (3)\;-39^{\circ} & \quad (4)\;+61.5^{\circ} \end {array}$$