Answer:
The total energy must be .7 J + .3 J = 1 J for a particle at the endpoint or midpoint of motion.
Also, omega = (k / m)^1/2
f = omega / (2 * pi)
omega^2 = 4 pi^2 * f^2 = k / m
k = 4 * pi^2 * f^2 * m = 40.05
Max KE or PE = 1/2 k A^2
A^2 = 2 * E / k = 2 * 1 / k = .0499
A = .223 meters