how to create a "persistent" variable definition?

Is it possible to create a "dependent" variable that is recalculated automatically when its "independent" variables are changed. For example, can I somehow define y = 3*x +5 such that y is instantly updated when x changes somewhere later in the code? As of now, I would have to re-write the "y=3*x+5" line of code after every time x *might* change.

I tried searching for this but I have no idea what to search for exactly.

Thanks very much!