Solve.
15x^4 - 28x^2 + 5 = 0
substitution should result in a quadratic equation in the variable u, ie
I let u = x^2
15u^4 - 28x^2 + 5= 0
15u^2 - 28u + 5 = 0
I used the quadratic formula
-(-28) + or - sqrt -28^2 - 4(15)(5) / 2(15)
28 + or - sqrt 784+300/30
28 + or - sqrt 1084/30
how'd I do?