We can't, because that is not the limit. If you intended to write
One of the first steps in checking the limit of a function at an x-value is seeing if the function itself is defined for that value, followed by whether it will be defined for nearby x-values.
In this case our function is:
Because this is a simple linear function, it is self evident that the function will be continuous throughout. Thus, the limit shall exist everywhere, and will be equal to the value
Knowing this, we plug in
This is not equal to the
It is possible that you mis-wrote the function, but for the function as you have written it, you cannot prove
If you instead meant to write:
You can perform the same process as above. Again, since the function is linear and continuous throughout, the function will be equal to its own limit at any point.