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.