There are two things you should do right away
Divide through by minus 3
Substitute another letter (say z) for x^2
Step One
-3x^4/-3 +27x^2/-3 + 1200/-3 = 0/-3
x^4 - 9x^2 - 400 = 0
Step 2
Let z = x^2
(x^2)^2 - 9(x^2) - 400 = 0
z^2 - 9z^2 - 400 = 0
Step 3
By factoring solve for z
(z - 25)(z + 16) = 0
z = 25
x^2 = 25
x = +/- 5
============
z + 16 = 0
z = - 16
x^2 = - 16
x = sqrt(-16)
x = +/- 4i
I've included a graph to show that the zeros are correct.
