Physics help again please?

Question

An ideal spring is used to fire a 15.0-g block horizontally. The spring has a spring constant of 20 N/m and is initially compressed by 7.0 cm. The kinetic energy of the pellet as it leaves the spring is:

zero

2.5 × 10–2 J

4.9 × 10–2 J

9.8 × 10–2 J

1.4 J

Answer 1

KE of spring = PE of spring

KE of spring = 1/2kx^2,

where k is the spring constant and x is the distance compressed

KE = 1/2(20)(.07)^2

KE = .049 J, which is the same as 4.9 x 10^-2 J

Answer 2

Use conservation of energy.  The kinetic energy of the block is equal to the potential energy of the spring.

We can find the potential energy of the spring by using

PE=(1/2) kx^2

Where

k=spring constant

x=distance compressed