The relation between magnetization and strength of the magnetic intensity, is $H=B\chi$, now use the formula of the magnetic intensity, due to solenoid, $B=nI$ and the relation between $\chi =\left({\mu }_r-1\right)$, 

$$\begin{gathered} \Rightarrow H=nI\times \left({\mu }_r-1\right) \\ \Rightarrow H=900\times 2.5\times \left(501-1\right)=1.125\times {10}^6 \mathrm{A} {\mathrm{m}}^{-1} \end{gathered}$$