I agree with Dawn, you can use a SEARCH formula to locate "300" in the string. I used a couple of formulas to solve a similar issue.
The first formula below looks for "REQ" in the field Req_Name and returns a number indicating what position the string begins in the field. The second formula pulls in the 10 characters where "REQ" begins and if it doesn't locate the string the field will be blank. Hope this helps.
=Search("REQ",Req_Name)
=If(StrPosREQ>0,Mid(Req_Name,StrPosREQ,10),""