Question 87832
I'm not sure whether you meant the problem to be find f(-2) given that
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{{{f(x) = 2^(4x-1)}}} <=== first case
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or 
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{{{f(x) = 2^(4x) - 1}}} <=== second case
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In either case, the way you work this problem is to substitute -2 for x and calculate the
answer.
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In the first case, substituting -2 for x results in:
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{{{f(-2) = 2^((4*(-2))-1) = 2^((-8 - 1)) = 2^(-9) = 1/2^9 = 1/512}}}
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In the second case, substituting -2 for x results in:
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{{{f(-2) = 2^(4*(-2)) - 1 = (2^-8) - 1 = 1/2^8 - 1 = 1/256 - 1 = -255/256}}}
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[Note that in these two problems {{{2^9 = 512}}} and {{{2^8 = 256}}}]
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Hope this helps you to see your way through the problem.  Just remember that when you
write f(-2) it means substitute -2 for the variable on the right side.  So f(5) would
mean substitute +5 for the variable, and so forth.