Answer
a. $ 3-x$
b. $(-\infty,2]$
Work Step by Step
Given $f(x)=x^2+1$ and $g(x)=\sqrt {2-x}, x\leq2$, we have:
a. $(f\circ g)(x)=(\sqrt {2-x})^2+1=3-x$
b. The domain of the above function can be found as $\{x|x\leq2\}$ or $(-\infty,2]$