Let $S _ { n }$ denote the sum of the first $n$ terms of the arithmetic sequence $\left\{ a _ { n } \right\}$. If $2 S _ { 1 } = 3 S _ { 2 } + 6$ , then the common difference $d = $ $\_\_\_\_$ .
Let $S _ { n }$ denote the sum of the first $n$ terms of the arithmetic sequence $\left\{ a _ { n } \right\}$. If $2 S _ { 1 } = 3 S _ { 2 } + 6$ , then the common difference $d = $ $\_\_\_\_$ .