for n => return [p1, p2] ; where n is an even integer and p1 and p2 are two prime integers that summate to n.

primes.rb

require 'prime'

# return two prime numbers that summate to n, where n is an even integer
def primes (n)
  if n % 2 != 0
    return "[ERROR] not an even number"
  end

  # identify primes
  prs = []
  for i in 0...n
    if Prime.prime?(i)
      prs.push(i)
    end
  end

  # find summation values
  for j in 0...prs.length
    for k in 0...prs.length
      if prs[j] + prs[k] == n
        return prs[j], prs[k]
      end
    end
  end

  return "[ERROR] something bad happened"
end

p primes(100) # [3, 97]
p primes(9)   # [ERROR] not an even number