Probability

When a certain biased die is rolled, a particular face occurs with probability \({1 \over 6} - x\) and its opposite face occurs with probability \({1 \over 6} + x\). All other faces occur with probability \({1 \over 6}\). Note that opposite faces sum to 7 in any die. If 0 < x < \({1 \over 6}\), and the probability of obtaining total sum = 7, when such a die is rolled twice, is \({13 \over 96}\), then the value of x is :