splitting field problem

x^4-5x^2+6 = (x^2 -3)(x^2 -2) =(x -rt3 )(x +rt3)(x -rt2)(x +rt2)

x^2 -3 and x^2 -2 are minimal irreducible polynomial over Q and both of them are of degree 2.
Hence, the splitting field is Q( rt3 , rt2) and the degree over Q is 2x2=4, where rt3 and rt2 are the positive square root of 3 and 2 respectively.