# = lim_ (h-> 0) ((x + h) ^ 2 + 9 (x + h) - 3 - (x ^ 2 + 9x -
# = lim_ (h-> 0) (x ^ 2 + 2xh + h ^ 2 + 9x + 9h - 3 - x ^ 2 - 9x + 3)
# = lim_ (h-> 0) (batalkan (x ^ 2) + 2xh + h ^ 2 + cancel (9x) + 9h - cancel (3) - cancel (x ^ 2) - cancel (9x))) / h #
# = lim_ (h-> 0) (2xh + h ^ 2 + 9h) / h #
# = lim_ (h-> 0) (h (2x + h + 9)) / h #
# = lim_ (h-> 0) (batalkan (h) (2x + h + 9)) / batalkan (h) #
# = lim_ (h-> 0) 2x + 0 + 9 #
= 2x + 9
