[tex]10 \sqrt{28} - 7 \sqrt{63} = {x}^{2} [/tex]
[tex]20 \sqrt{7} - 21 \sqrt{7} = {x}^{2} [/tex]
[tex] - \sqrt{7} = {x}^{2} [/tex]
[tex]x = \sqrt[4]{7} \: \: sau \: - \sqrt[4]{7} [/tex]
