Find all the zeroes of the equation. -3x4+27x2+1200=0 i need to learn how to do it! just not the answer

The first thing you should do for this case is to rewrite the expression.
 To do this, divide both sides of the equation between -3.
 We have then:
 (-1/3) * (- 3x ^ 4 + 27x ^ 2 + 1200) = (- 1/3) * (0)
 x ^ 4-9x ^ 2-400 = 0
 Then, we rewrite the polynomial:
 (x-5) * (x + 5) * (x ^ 2 + 16) = 0
 From here, we obtain the four roots:
 Root 1:
 x-5 = 0
 x = 5
 Root 2:
 x + 5 = 0
 x = -5
 Root 3 and root 4:
 x ^ 2 + 16 = 0
 x3 = -4i
 x4 = 4i

 
 all the zeroes of the equation are: 
 x1 = 5 
 x2 = -5
 x3 = -4i
 x4 = 4i