Prefix as a Substring

Given $3$ string $S_1$, $S_2$ and $X$, find the number of non-empty substrings of $X$ which can expressed as $P + Q$ i.e. concatenation of strings $P$ and $Q$, where $P$ is some prefix of $S_1$ (possibly empty) and $Q$ is some prefix of $S_2$ (possibly empty). A **substring** of a string is a contiguous subsequence of that string. For example, "chef" is a substring of "codechef", but "def" is n