Question

find all real numbers x that x^4+6x^2-72=0

Answer 1

±√6

Answer 2

If x^2 = y
then y^2+6y-72=0
so y^2+12y-6y-72=0
thus y(y+12)-6(x+12)=0
therefore (y+12)(y-6)=0

since y+12 = 0 yields negative values of y:
only y - 6 = 0 is true
replacing y = x^2
x^2 - 6 = 0
x^2 = 6
x = + or - sqrt(6)