Guys, please learn how to properly present your questions so that persons you are seeking to help you would be able to adequately understand what actually you're asking, OK?

Now for this task: here we have an equations' system with 2 equations and 2 variables. Let's x be the length of the one part of the cable, y - the length of the rest part. Hence the general length is

**x + y = 84**. Since one part is 18 m longer that the other so **x = y + 18 **or **y = x + 18**. Inserting the expression for any of these variables into the very first equation and solving it for **y** or **x** respectively we then can find the other variable ( **x** or **y** respectively)

solve(x + y = 84, x = y + 18)