Write and test a new function common$\_$substring $,$ with arguments left and right as in part $($a$)$ but also accepting an additional integer argument length. The function should check whether any substring of length characters in left matches a substring of length characters in right. The function should return the first matching substring of length characters or the empty string if there is no match.

Complete the function in the following cell. We have provided some tests $-$ your tutor may use additional tests.