Firstly, you need to understand the meaning of B.C.E. and C.E. B.C.E. means "Before common era", where "common era" is the year where Jesus Christ was born (I will just call it "The Year" since the name is so long). The date has been used as a dating system for many years.
In this case, 300 B.C.E. means 300 years before "The Year" whereas 250 C.E. means 250 years after "The Year". Thus, the total would be:
