Can We multiply a constant in a proportionality?

Question

If we have a proportionality stating y is proportional to x can we say y is proportional to maybe 2x or kx in general? Because if y is proportional to x and x in turn is a function say 5z then y is proportional to 5z. But the problem arises if we put in the constant term to be 0, then a variable is proportional to a constant. I understand that this is a radical though but it hit me while doing physics but I thought it is more of a Math problem