What is the amplitude of oscillation in cm

Question

A 150 gram ball attached to a spring with spring constant

2.30 N/m oscillates horizontally on a frictionless table. Its velocity is 20.0 cm/s when x= -5.00 cm.

What is the amplitude of oscillation in cm?

What is the speed of the ball when x= 3.00 cm? answer in cm/s

Answer 1

E = 0.5mv(t)^2+0.5kx(t)^2) = 0.005875 J

A = xmax = sqrt(2E/k) = 0.071475 m

v = ±sqrt(2(E-0.5kx(t)^2)/m) = ±0.254034 m/s when x(t) = 3 cm