The first differential of a linear function like yours is a constant, which is independent of x. The instantaneous time rate of change is the same as all other time rates of change. The answer is 3.
If you're asking how to find the rate of change by first principles then it's this:
Then is the exact same equation as above, only instead of x, you have x+h, so:
Applying this to the definition of a first differential:
Or by plugging in x = 2: