Solving Simple Pendulum Homework: Find the Amplitude & Phase Angle

[cdotter]

Homework Statement

A 2.00-kg frictionless block is attached to an ideal spring with force constant 300 N/m. At t=0 the spring is neither stretched nor compressed and the block is moving in the negative direction at 12.0 m/s. Find the amplitude and the phase angle.

Homework Equations

x=A*cos(ωt+φ)
v=-ωA*sin(ωt+φ)
\omega = \sqrt{\frac{k}{m}}

The Attempt at a Solution

\omega = \sqrt{\frac{300}{2.00}} = 12.25
0 = A\cdot cos(\phi)
12.0 = -12.25 \cdot A \cdot sin(\phi)

How do I solve for something when there are two unknowns (phi and A)?

 

[cdotter]

Do I assume that A is non-zero, therefore making φ = Pi/2 radians?