Use the substitution
we have the equation x^4 - 5x^2 + 4 and to find when the values of x give us 48 we do this : x^4 - 5x^2 + 4 = 48, then we try to factor. In this case we can not, so do we use the quadratic formula? The answers in the text book are 48 and -3.10 ? They are wrong... So what do i do?