The power of x of following must be 10.
(x0 + x1 + ...+ x10)(x0 + x1 + ...+ x5)(x0 + x1 + ...+ x3)
= $\frac{1-x^{11}}{1-x}$ . $\frac{1-x^{6}}{1-x}$ . $\frac{1-x^{4}}{1-x}$
Now, we neglect the x11 power.
= (1 - x4 - x6 + x10) . (1 - x)-3
= (1 - x4 - x6 + x10) . ( 66x10 + 28x6 + 15x4 + 1)
= 24x10