To solve for x, we first need to find a common denominator for 10 and 15, which is 30.

[ \frac{3}{10} \times \frac{3}{3} = \frac{9}{30} ]

[ \frac{4}{15} \times \frac{2}{2} = \frac{8}{30} ]

Now we can rewrite the equation:

[ \frac{9}{30} - \frac{8}{30} = \frac{x}{12} ]

[ \frac{1}{30} = \frac{x}{12} ]

To solve for x, we multiply both sides by 12:

[ 12 \times \frac{1}{30} = x ]

[ \frac{12}{30} = x ]

[ \frac{2}{5} = x ]

Therefore, x = 2/5.