Ответ:

Пo теореме косинусов имеем

[tex]\bf x^2=5^2+7^2-2\cdot 5\cdot 7\cdot cos60^\circ \\\\x^2=74-70\cdot \dfrac{1}{2}\\\\x^2=74-35\\\\x^2=39\\\\x=\sqrt{39}[/tex]