A small particle of mass 2.0 x 10^-19 kg is attached to a string of relaxed length 0.50 nm .The string is the black horizontal line attached to a an immovable wall on the left. The plot is a graph of the force needed to stretch the string by pulling to the right on it .The length of the string is x.

Question

A small particle of mass 2.0 x 10^-19 kg is attached to a string of relaxed length 0.50 nm .The string is the black horizontal line attached to a an immovable wall on the left. The plot is a graph of the force needed to stretch the string by pulling to the right on it .The length of the string is x.
a)  Calculate the force constant-the “spring constant” k for this string.
b)  Assume that the string is stretched to a total length of x = 3.0 nm and released from rest. Then the particle will move back and forth along the x direction. Use your k from part a to compute the maximum speed of the particle as it oscillates back and forth. Can some one show me how to put this in a formula and solve?