[tex]3 \sqrt{5} -\sqrt{20}+2\sqrt{45}
3*5^{1/2} -20^{1/2}+2*45^{1/2
20^{1/2}=(2^{2}*5)^{1/2}=2*5^{1/2}
3*5^{1/2}-2*5^{1/2}+2*45^{1/2}
45^{1/2}=(3^2*5)^{1/2}=3*5^{1/2}
3*5^{1/2}-2*5^{1/2}+2*3*5^{1/2}
3*5^{1/2}-2*5^{1/2}+6*5^{1/2}
6*5^{1/2}+5^{1/2}*(3-2)
6*5^{1/2}+5^{1/2}
5^{1/2}*(6+1)
7*5^{1/2}
7\sqrt{5} [/tex]