How Do You Calculate the Amplitude of Oscillation for a Spring-Mass System?

[kristibella]

Homework Statement

A 200 g ball attached to a spring with spring constant
2.50 N/m oscillates horizontally on a frictionless table. Its velocity is 18.0 cm/s when x = -5.00 cm.
A. What is the amplitude of oscillation?

B. What is the speed of the ball when x = 3.00 cm?

Homework Equations

A. $$\omega$$ = sqrt k/m = sqrt 2.50/.200
v(t) = A*$$\omega$$*sin($$\omega$$t)
t = 0
v(t) = A$$\omega$$

B. V_x² = $$\omega$$²(A² - x²)

The Attempt at a Solution

I've tried several times to get a solution and I have never gotten the same answer twice...
A. $$\omega$$ = 3.54
A = 5.09 cm

B. When x = 3.00 cm, V = 14. 56 cm.

Neither of these answers are correct and I'm not sure what I am doing wrong. I am doing everything as explained to me in class. I'm so confused...

 

[Redbelly98]

Have you been taught about conservation of energy, and the potential energy of a spring?

Also, is there supposed to be a question in part A? I only see a question in B.