Just remember to use the Binomial Theorem.

The binomial Theorem tells us that $$\begin{aligned}(x-3)^{10} &=  \sum_{k = 0}^{10} \binom{10}{k} x^{n-k} \cdot (-3)^k \\ &= \binom{10}{0} x^{10} + \binom{10}{1} x^{9}\cdot (-3) + \cdots + \binom{10}{9} x\cdot (-3)^9 + \binom{10}{10} (-3)^{10}\end{aligned}$$ so the term with $x^7$ in the expansion of $(x - 3)^{10}$ is $\displaystyle \binom{10}{3} x^7 \cdot (-3)^3$, so the coefficient of $x^7$ is $\displaystyle \binom{10}{3} \cdot (-3)^3 = -3240$.