Question 500216
x^4 -7x^2 + 12= 0
x^4 -3x^2 - 4x^2 +12 = 0
x^2(x^2 -3) -4(x^2-3) = 0
(x^2 - 4) (x^2 -3) = 0
=> x^2 = 4 or x^2 = 3
=>x = +2 or -2 
  x = +{sqrt(3)) or -{sqrt(3)}