The relationship in this equation is linear, but not proportional. If it was proportional, then replacing x with nx should give ny as the answer. Example:
y = 3x
Replace x with 2x.
y' = 3(2x) = 2(3x) = 2y
y' = 2y
This is proportional. But if you replace x with 2x in
y = 3x + 5,
you will not get 2y for y'. This case is not proportiona