Answer
$-\frac{21}{2}\cdot x^6$
Work Step by Step
According to the Binomial Theorem we can obtain the $r+1$th term of the expansion of the binomial $(x+y)^n$ by the formula $_nC_rx^{n-r}y^r$.
Hence here it is: (by plugging in $n=9,r=3$ and $x,-\frac{1}{2}$: $_{9}C_3(x^3)^{9-3}(-\frac{1}{2})^3=84\cdot x^6\cdot -\frac{1}{2}=-\frac{21}{2}\cdot x^6$
