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This problem is not about Algebra. It is about arithmetic.
If Peter is 4 years now, twice his age is 8 years.
Hence, In 5 years, Simon will be 8 years old.
It means that now Simon is 8-5 = 3 years old.
You don't need to use equation to solve such simple, elementary problem (!)
It is, probably, a second grade's level problem . . .