For reaction a A arrow x P , when [A]=2.2 mM , the rate was found...

$a A \rightarrow x P$ Rate of reaction $=[A]^a$ Order of reaction $=a$ $[A]_1=2.2 \mathrm{mM}, r_1=2.4 \mathrm{mM} \mathrm{s}^{-1}$ $[A]_2=\frac{2.2}{2} \mathrm{mM}, r_2=0.6$ or $\frac{2.4}{4} \mathrm{mM} \mathrm{s}^{-1}$ On reducing the concentration of $A$ to half, the rate of reaction is decreased by four times. $\therefore$ Rate of reaction $\propto[A]^2$ $\Rightarrow$ Order of reaction $=2$