1) b (b^3)^4 / b^9 = b*b^12 / b^9 = b^(1+12-9) = b^4
2) 9x^2 y^3 - x^2 y^3 -10 x^2 y^3 = x^2 y^3 (9 - 1 -10) = -2 x^2 y^3
3) (3 x^2 y)^4 (3 xy^3)^2 = 3^4 x^(2*4) y^4 * 3^2 x^2 y^(3*2) = 3^(2+4) x^(8+2) y^(4+6) = 3^6 x^10 y^10 = 729 x^10 y^10
4) (c^4)^5 c^8 / (с^7)^4 = c^(4*5 + 8) / c^(7*4) = c^28/c^28 = 1
5) 21^12 / ((7^4)^3 * (3^2)^4) = 3^12 * 7^12 / (7^12 * 3^8) = 3^12 / 3^8 = 3^(12-8) = 3^4 = 81