Okay everyone Last Question I have today I promise!

Given: A 2.1-kg mass attached to a spring oscillates with an amplitude of 9.0 cm and a frequency of 4.0 Hz.
Questions:What is its energy of motion?

This is what I have so far. E= K+U
so; (1/2)mv^2+(1/2)kx^2
v=0 so we are left with E=(1/2)kx^2
I can find K by using T=2(pi)*sqrt of m/k
frequency is 1/T. Thus I set 4.0=1/T and get T=.25
I then set .25=T=2(pi)*sqrt of m/k
Plug in what we know: .25=2(pi)*sqrt of 2.1/k
I then solve for k. I divide .25 by 2(pi) ,square root both sides and divide 2.1 by .154213 and get k=13.6.
Is this right? I think I am making some error along the way.
When I plug that into (1/2)kx^2 to get E. It says that answer is wrong.
What am I doing wrong?

You were oK up through here:



From this you should get:



Always check your units to make sure you haven't messed something up.