How can you show that two objects are proportional with an equation

You can show that (y) is proportional to (x) by writing:

This would mean that:

Whereby (k) would be the constant of proportionality.

This value can be an element of a real number.

k∈R

What are real numbers?

Integers, rational numbers and irrational numbers.

Example of a graph which would show that 2 objects are proportional:

y=2x

What is the inequality?