3x^y*7x^5b^2c^4

3x^4 y * 7x^5 b^2 c^4 = (3 * 7) * (x^4 * x^5) * (y * b^2 * c^4) = 21 * x^(4 + 5) * (yb^2 c^4) = 21 * x^9 * (yb^2 c^4) = 21x^9 * (yb^2 c^4) = 21x^9 yb^2 c^4

Ответ: 21x^9 yb^2 c^4.