3. Given the set $M=\left\{(x, y) \mid y=\sqrt{9-x^{2}}\right\}, N=\{(x, y) \mid y=x+b\}$, and $M \cap N=\varnothing$, find the range of real number $b$.

 untranslated text:
已知集合 $M=\left\{(x, y) \mid y=\sqrt{9-x^{2}}\right\}, N=\{(x, y) \mid y=x+b\}$, 且 $M \cap N=\varnothing$, 求实数 $b$ 的取值范围.
3. Solution: Set $M$ represents the upper semicircle (including endpoints) centered at the origin with a radius of 3, and set $N$ represents a line with a slope of 1. Therefore, we can get $b>3 \sqrt{2}$ or $b<-3$.