$\text{Percentage decrease }= \dfrac{\text{actual decrease}}{\text{original value}} \times 100$
- Decrease between $2003$ and $2018 = 5.5 \cdot 10^6 \text{ km}^2 – 4.6 \cdot 10^6 \text{ km}^2 = 0.9 \cdot 10^6 \text{ km}^2$
- Percentage decrease $2003$ of $2018$ value $=\dfrac{ 0.9 \cdot 10^6 \text{ km}^2}{4.6 \cdot 10^6 \text{ km}^2} = 19.6\%$
Well I assume you are looking for the following:
The decrease of land between $2003$ and $2018$ is $5.5 \times 10^6 \text{ km}^2-4.6\times 10^6 \text{ km}^2=0.9\times10^6 \text{ km}^2$.
Therefore the percentage decrease is $(\frac{0.9\times 10^6}{5.5 \times 10^6}) =16.4\%$.
The percentage decrease is $\frac{\text{amount of decrease}}{\text{original value}} \times 100$ by definition.
Here your original value is the area of land in $2003$ that is $5.5\times 10^6 \text{ km}^2$