Hi there!
We can use the conservation of energy:
Ei = Ef
The initial energy involves kinetic energy while the final energy involves maximum elastic potential energy, so:
1/2mv² = 1/2kx²
We can cancel out the 1/2s for ease of solving:
mv² = kx²
Plug in given values to solve for x:
√(mv²/k) = x
√(5.7)(12²)/(150)) = x
x = 2.339 m