Answer
The work that has been done on the block by the applied force is $~~0.18~J$
Work Step by Step
The block will stop when the magnitude of the applied force is equal in magnitude to the opposing force on the block by the spring.
We can find $x$:
$kx = F$
$x = \frac{F}{k}$
$x = \frac{3.0~N}{50~N/m}$
$x = 0.060~m$
We can find the work that has been done on the block by the applied force:
$W = F~x$
$W = (3.0~N)(0.060~m)$
$W = 0.18~J$
The work that has been done on the block by the applied force is $~~0.18~J$.