The formula is correct, but how you relate H to µr given in the question? You also forgot to give the complete core dimensions. You gave x size, but y size is missing, thus we can't calculate magnetic path length l.these the only information provided in the question. The formula says H = I*N/l
where is H , magnetic field strength
I current in the coil
N , number of turns
l the length of the metal circuit
these the only information provided in the question. The formula says H = I.N/I