(2целых2/3х+1целая3/7х)*21=58

(2 2/3 * х + 1 3/7 * х) * 21 = 58;

((2 * 3 + 2)/3 + (1 * 7 + 3)/7) * х = 58/21;

(8/3 + 10/7) * х = 58/21;

(7 * 8/21 + 3 * 10/21) * х = 58/21;

(56/21 + 30/21) * х = 58/21;

(56 + 30)/21 * х = 58/21;

86/21 * х = 58/21;

х = 58/21 : 86/21 = 58/21 * 21/86 = 58/86 = 29/43;

Ответ: х = 29/43.