Answer:
a = 9 or a = 1/3
f(x)=3x^2-28x
f(a)=3x^2-28x
-9=3x^2-28x
3x^2-28x+9=0
by factory,
(3x-1)(x-9)=0
3x=1 or x=9
x=1/3 or x=9
x=a
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