Let f$($x$)=2$x$6$x$2+6.$ Then according to the definition of derivative

f$($x$)=$limh$0$f$($x$+$h$)$f$($x$)$

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h

$($Your answer above and the next few answers below will involve the variables x and h$.$ We are using h instead of $$x because it is easier to type$)$

We can cancel the common factor h from the numerator and denominator leaving the polynomial

$212$x$6$h

Taking the limit of this expression gives us f$($x$)=212$x