For each of the given functions f(x), find the derivative (f 1) (c) at the given point c, first finding a = f (c). a) f(x) = 5x + 7x ;c= -12 a = ( f - 1 )' ( c ) = b) f(ac) = x2 - 12x + 43 on the interval [6, co); c = 8 a = ( f - 1)' ( c ) = 0