Answer:
coefficient of u^2 is = -9
Step by step:
using identity; \left(a-b\right)^3=a^3-b^3-3a^2b+3ab^2
\left(u-3\right)^3=\left(u\right)^3-\left(3\right)^3-3\left(u\right)^2\left(3\right)+3\left(u\right)\left(3\right)^2
\left(u-3\:\right)^3\:=\:\:u^3-27-9u^2+27u
so coefficient of u^2 is = -9