For real numbers a,b (a > b > 0), let Area $\left\{(\mathrm{x},\mathrm{y}):{\mathrm{x}}^2+{\mathrm{y}}^2\le {\mathrm{a}}^2 \mathrm{and} \frac{{\mathrm{x}}^2}{{\mathrm{a}}^2}+\frac{{\mathrm{y}}^2}{{\mathrm{b}}^2}\ge 1\right\}=30\mathrm{\pi }$ and $\mathrm{Area} \left\{(\mathrm{x},\mathrm{y}):{\mathrm{x}}^2+{\mathrm{y}}^2\ge {\mathrm{b}}^2 \mathrm{and} \frac{{\mathrm{x}}^2}{{\mathrm{a}}^2}+\frac{{\mathrm{y}}^2}{{\mathrm{b}}^2}\le 1\right\}=18\mathrm{\pi }$  Then the value of (a-b)² is equal to__.