It says i have plus subscripton but whenever i log onto using my laptop, it says i don't have plus and whenever i try to ''purchase it'', it says i already paid for it??? like i'm genuinely confused.

For positive constants a and b, the force between two atoms in a molecule is given f(r) = −a r2 + b r3 , where r > 0 is the distance between the atoms. note: a and b are upper case letters. (a) find f '(r) = (b) find the critical point for f(r). r = (c) find f ''(r) = (d) find the value of r so that f ''(r) = 0.