Given f(x)= 8 − x^2, solve f(3x) = -28
First substitute (3x) into the equation for f(x), giving f(3x) = 8 - 9x2. This is then equal to -28 and so it is remains to solve the equation 8 - 9x2 = -28. This can be written as 9x2 = 36, dividing through by 9 gives x2 = 4 and so x = ±2.
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