Amplitude Question Just Need A Formula

[MJC8719]

At t = 0, a 730 g mass at rest on the end of a horizontal spring (k = 128 N/m) is struck by a hammer, which gives it an initial speed of 2.74 m/s.
(a) Determine the period of the motion.
s
Determine the frequency of the motion.
Hz
(b) Determine the amplitude.
m

I have found parts A with no problem...but i was hoping someone could post the formula that I can use to find the amplitude as I am having some real trouble figuing it out.

 

[G01]

Please show some work. You won't get help until you do, according to the forum guidelines. (I don't mean to be rude, but it's the only way we can be sure the person asking for help doesn't just want someone else to do their homework.)