Construct regular expression that contains the substring
Any string containing $abcba$ is of the form $xabcbay$, where $x,y$ are strings. This string contains an even number of $b$'s iff $xy$ contains an even number of $b$'s iff either $x,y$ both contain an even number of $b$'s, or both contain an odd number of $b$'s. This leads to the following regular expression:
$$
(a+c)^*(b(a+c)^*b(a+c)^*)^*abcba(a+c)^*(b(a+c)^*b(a+c)^*)^* + \\
(a+c)^*b(a+c)^*(b(a+c)^*b(a+c)^*)^*abcba(a+c)^*b(a+c)^*(b(a+c)^*b(a+c)^*)^*
$$