Answer
$x = 0.060~m$
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 $x$:
$kx = F$
$x = \frac{F}{k}$
$x = \frac{3.0~N}{50~N/m}$
$x = 0.060~m$
