I typed your equation into the file on my machine using 2017 SP3.0. It seems to work fine:
I noticed that your file is in version 2013 though. If you have recently installed this version of SOLIDWORKS on your machine (in the last 2 years) then you would be encountering a known issue with equations due to a Windows Update released in July 2015. To fix this, just follow the instructions detailed here (GoEngineer - How to Fix Equation Syntax Issue).