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[tex]x^2=7^2+(3\sqrt3)^2\\\\x^2=49+3^2\cdot(\sqrt3)^2\\\\x^2=49+9\cdot3\\\\x^2=49+27\\\\x^2=76\\\\x=\sqrt{76}=\sqrt{4\cdot19}=\boxed{2\sqrt{19}}[/tex]
