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?