Chemistry Examples

Find the Unknown in the Combined Gas Law Determine (the)(total)(pressure)(of)a(gas)(mixture)(that)(contain)(oxygen)(nitrogenandhelium)(if)(the)(pressures)(of)(the)(gasses)(are):P(O)=150(mm)Hg(P(N))=350(mmHg)(P(He))=200(mm)Hg
I am unable to solve this problem.