it works in Ruby Console for me…
pt1 = [50, 65, 0]
pt1a = [pt1.x + 5, pt1.y + 5, pt1.z - 5]
# => [55, 70, -5]
# I would more likely use
pt1 = [50, 65, 0]
a = [5, 5, - 5]
pt1a = pt1.zip(a).map(&:sum)
# => [55, 70, -5]
# same result but easier to apply to any array...
what, where and how are you getting an error?
john