# = lim_ (h-> 0) (x + h) ^ 2 + 9 (x + h) - 3 - (x ^ 2 + 9x - 3)) / h #
# = lim_ (h-> 0) (x ^ 2 + 2xh + h ^ 2 + 9x + 9h - 3 - x ^ 2 - 9x + 3) / h #
# = lim_ (h-> 0) (annuller (x ^ 2) + 2xh + h ^ 2 + afbryd (9x) + 9h - annuller (3) - annuller (x ^ 2) - annuller (9x) + annuller)) / h #
# = lim_ (h-> 0) (2xh + h2 2 + 9h) / h #
# = lim_ (h-> 0) (h (2x + h + 9)) / h #
# = lim_ (h-> 0) (annuller (h) (2x + h + 9)) / annuller (h) #
# = lim_ (h-> 0) 2x + 0 + 9 #
= 2x + 9
