if f(x)=x^(2)+4x, find f(s+6)

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In other words, they want the value of f(x) when x = s + 6.  So wherever there is an x in f(x), we need to substitute s + 6:      f(x) = x^2 + 4xf(s + 6) = (s + 6)^2 + 4(s + 6)            =(s + 6)(s + 6) + 4(s + 6)            = (s^2 + 12s + 36) + 4s + 24            = s^2 + 16s + 60