a) $\sqrt{20}$ -$\sqrt{45}$ +3$\sqrt{18}$ +$\sqrt{72}$
=$2\sqrt{5}$ -$3\sqrt{5}$ +$3.3\sqrt{2}$ +$6\sqrt{2}$
=$2\sqrt{5}$ -$3\sqrt{5}$ +$9\sqrt{2}$ +$6\sqrt{2}$
=$-\sqrt{5}$ +$15\sqrt{2}$
b) $\sqrt{48}$ -$2\sqrt{75}$ +$\sqrt{108}$
=$4\sqrt{3}$ -$2.5\sqrt{3}$ +$6\sqrt{3}$
=$4\sqrt{3}$ -$10\sqrt{3}$ +$6\sqrt{3}$
=0