Answer :

[tex]x^2-10x=39\\\\x^2-2\cdot x\cdot5=39\ \ \ \ /+5^2\\\\\underbrace{x^2-2\cdot x\cdot5+5^2}_{(*)}=39+5^2\\\\(x-5)^2=39+25\\\\(x-5)^2=64\iff x-5=\pm\sqrt{64}\\\\x-5=-8\ or\ x-5=8\\\\x=-8+5\ or\ x=8+5\\\\x=-3\ or\ x=13[/tex]

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