when you see "from first principles" they are talking about using the definition of the derivative.

there are two (equivalent) definitions:

if this is the definition you have been taught, then you are required to find the limit

alternatively, you may use the definition

in this case you are required to find the limit

(and replace with when finished)

Notation: and mean "the derivative of (with respect to )" and "the derivative of (with respect to x) at the point " respectively.

here of course,