Spring force and frictionless mass vs speed

[wind]

Homework Statement

A mass
(m = 1.35 kg),
originally at rest, sits on a frictionless surface. It is attached to one end of an unstretched spring
(k = 747 N/m),
the other end of which is fixed to a wall (see figure below). The mass is then pushed with a constant force to stretch the spring. As a result, the system (which consists of the wall, spring and mass) comes to a momentary stop after the mass moves 15.0 cm. Find the following.(e) speed of the mass 4.50 cm into the motion

Homework Equations

The Attempt at a Solution

I tried 1/2mv^2 = 1/2K (x1^2-x2^2)
v= sqrt(K ( (0.15^2-0.045^2))/m

not sure how to solve this .. been at it for a while now.

 

[CWatters]

I haven't tried to solve it but... when the mass comes to a momentary halt you know all the applied energy is stored in the spring.

 

[BvU]

Hello wind, and welcome to PF.

Your attempt is based on an equation for energy conservation.
You might need another term. How about something that features F ?