Answer
The temporal coordinate of the event according to S' is $~~t' = 2.29~s$
Work Step by Step
We can find $\gamma$:
$\gamma = \frac{1}{1-\beta^2}$
$\gamma = \frac{1}{1-0.400^2}$
$\gamma = 1.09$
We can find the temporal coordinate $t'$:
$t' = \gamma~(t-vx/c^2)$
$t' = 1.09~[(2.50~s)-\frac{(0.400)(3.0\times 10^8~m/s)(3.00\times 10^8~m)}{(3.0\times 10^8~m/s)^2}]$
$t' = 2.29~s$
The temporal coordinate of the event according to S' is $~~t' = 2.29~s$
