Two charges are placed on the $x$ axis. One of the charges $\left(q_{1}=\right.$ $+8.5 \mu \mathrm{C})$ is at $x_{1}=+3.0 \mathrm{cm}$ and the other $\left(q_{2}=-21 \mu \mathrm{C}\right)$ is at $x_{1}=+9.0 \mathrm{cm}$
Find the net electric field (magnitude and direction) at
(a) $x=0 \mathrm{cm}$ and
(b) $x=+6.0 \mathrm{cm}$