Nicotine leaves the body at a constant rate. At a time of $2$ hours after smoking a cigarette, $52$ mg of nicotine remain in a person's body; $7$ hours later ($9$ hours after smoking the cigarette), there are $13$ mg of nicotine in the body.

(a) Find a formula for the amount of nicotine $A$ left in the body $t$ hours after smoking a cigarette.

$A = f(t) =$ help (formulas)

(b) How much nicotine is in the body immediately after smoking.

help (numbers)

(c) How much nicotine is in the body $6$ hours after smoking.

help (numbers)