b) $C = \sqrt{11} -\sqrt{(3 -\sqrt{11})^2}$
$\to C =\sqrt{11} - |3 -\sqrt{11}|$
$\to C = \sqrt{11} -(\sqrt{11} -3)$
$\to C =\sqrt{11} -\sqrt{11} +3$
$\to C = 3$
c) $\sqrt5 -4\sqrt{20} + 2\sqrt{45}$
$=\sqrt5 -4\sqrt{4.5}+2\sqrt{9.5}$
$=\sqrt5 - 8\sqrt5 + 6\sqrt5$
$= -\sqrt5$