a)
6\sqrt8 + 7\sqrt{28} - 2\sqrt{18} - 3\sqrt{63}=6\sqrt{4*2}+7\sqrt{4*7}-2\sqrt{9*2}-3\sqrt{9*7}=
6*2\sqrt2+7*2\sqrt7-2*3\sqrt2-3*3\sqrt7=12\sqrt2+14\sqrt7-6\sqrt2-9\sqrt7=6\sqrt2+5\sqrt7
b)
5\sqrt{98} - 3\sqrt{45} + 2\sqrt{128} - 6\sqrt{180}=
5\sqrt{49*2}-3\sqrt{9*5}+2\sqrt{64*2}-6\sqrt{36*5}=
5*7\sqrt2-3*3\sqrt5+2*8\sqrt2-6*6\sqrt5=
35\sqrt2-9\sqrt5+16\sqrt2-36\sqrt5=
51\sqrt2-45\sqrt5