Решение:
15\(\frac{4}{19}\) = \(\frac{15 \times 19 + 4}{19}\) = \(\frac{285 + 4}{19}\) = \(\frac{289}{19}\)
3\(\frac{17}{19}\) = \(\frac{3 \times 19 + 17}{19}\) = \(\frac{57 + 17}{19}\) = \(\frac{74}{19}\)
21\(\frac{2}{19}\) = \(\frac{21 \times 19 + 2}{19}\) = \(\frac{399 + 2}{19}\) = \(\frac{401}{19}\)
\(\frac{289}{19}\) + x + \(\frac{74}{19}\) = \(\frac{401}{19}\)
\(\frac{289 + 74}{19}\) + x = \(\frac{401}{19}\)
\(\frac{363}{19}\) + x = \(\frac{401}{19}\)
x = \(\frac{401}{19}\) - \(\frac{363}{19}\)
x = \(\frac{401 - 363}{19}\)
x = \(\frac{38}{19}\)
x = 2
Ответ: 2