Answer
$y = -21.6~m$
Work Step by Step
We can find the electrostatic force in unit-vector notation:
$F = q~E$
$F = (8.00\times 10^{-5}~C)~(3000~\hat{i} - 600~\hat{j})~N/C$
$F = (0.240~\hat{i} - 0.048~\hat{j})~N$
We can find the acceleration in unit-vector notation:
$F = ma$
$a = \frac{F}{m}$
$a = (\frac{1}{0.0100~kg})\cdot (0.240~\hat{i} - 0.048~\hat{j})~N$
$a = (24.0~\hat{i} - 4.80~\hat{j})~m/s^2$
We can find the y coordinate at $t = 3.00~s$:
$y = \frac{1}{2}a_yt^2$
$y = \frac{1}{2}(-4.80~m/s^2)(3.00~s)^2$
$y = -21.6~m$
