Given:
k = 350 N/m, the spring constant
E = 1200 J, the energy stored in the spring
The force in the spring, when stretched by x, is
F = k*x
The work done by stretching the spring (energy stored in the spring) is
W = (1/2)*F*x = (1/2)kx²
Therefore
(1/2)*(350 N/m)*(x m)² = 1200 J
175x² = 1200
x² = 6.857
x = 2.619 m
Answer: 2.62 m (nearest hundredth)