What's the equivalent of the EXCEL MID FUNCTION SQL Server 2008

Hi Experts,
I have the following function in EXCEL (see below), how can I write this same function in SQL Server 2008 R2?
I don't believe there is a MID function in SQL.  What do I use instead?

=(MID(C4,1,1)*3)+(MID(C4,2,1))+(MID(C4,3,1)*3)+(MID(C4,4,1))+(MID(C4,5,1)*3)+(MID(C4,6,1))+(MID(C4,7,1)*3)+(MID(C4,8,1))+(MID(C4,9,1)*3)+(MID(C4,10,1))+(MID(C4,11,1)*3)+(MID(C4,12,1))+(MID(C4,13,1)*3)+(MID(C4,14,1))+(MID(C4,15,1)*3)+(MID(C4,16,1))+(MID(C4,17,1)*3)

Thank you very much in advance,
mrotor