A two pan scale is unbalanced. If a sack of sugar is put over the right pan, the sugar weighs $19\,kg$. If it is put on the left pan, it weighs $15\,kg$. What is the weight of the sack of sugar?

The alternatives given are as follows:

$\begin{array}{ll}

1.&17\,kg\\

2.&16\,kg\\

3.&18\,kg\\

4.&15\,kg\\

\end{array}$

What I've attempted to do was to make this equation:

$\textrm{left pan + sugar = 15}$

$\textrm{right pan + sugar = 19}$

$\textrm{right pan - left pan = 4}$

But in this given situation I'm not given any additional equation to solve this riddle. How exactly can I find this unknown weight? Help please?