entity framework - EF other way to Sum() new Column -

i got stuck on query calculate new column. cannot explain briefly see snippet code below.

from user in context.table select new  {     total = user.total,     paid = user.paid,     balance = //should total - paid assign result }  

i have tried query

var result = in context.enrollmentrequests                          a.schoolyear == schoolyear                          select new                          {                              a.studentid,                              name = a.student.firstname + " " + a.student.middlename + " " + a.student.lastname,                              tuition = context.miscs.where(m => m.yearlevel == a.yearlevel && m.schoolyear == schoolyear && m.term == a.term && m.courseid == a.courseid)                                                 .select(ms => new { amount = ms.amount })                                                     .union(context.studentcharges                                                         .where(s => s.yearlevel == a.yearlevel && s.schoolyear == schoolyear && s.term == a.term && s.courseid == a.courseid && s.studentid == a.studentid)                                                             .select(ss => new { amount = ss.amount }))                                                                 .union(context.studentsubjecttakes                                                                     .where(st => st.studentid == a.studentid && st.schoolyear == a.schoolyear && st.term == a.term && st.yearlevel == a.yearlevel && st.educationallevel == a.student.studentadvanceeducations.firstordefault().educationlevel)                                                                         .select(st => new                                                                         {                                                                             amount = context.subjectofferedfees                                                                                 .where(f => f.subjectsofferedid == st.subjectsofferedid).sum(w => (decimal?)w.cost ?? 0)                                                                         }))                                                                                 .select(f => f.amount).sum(),                              paymentmade = context.payments.where(p => p.schoolyear == schoolyear && p.term == a.term && p.studentid == a.studentid && p.paymentdes == "tuition fee").sum(sm => (decimal?)sm.amount),                              balance = tuition - paymentmade //does not exist on current context                          }; 

but doesn't work says not exist on current context.

how possible. thanks. helpful anyone.

balance = user.total - user.paid