Quote by melissa_y
Ok so this is what I've done and I am not gettingthe problem right still....
I did T = 2pi (square root of L/g)
I took that T/2 = 5.5875
Then i set up this equation
5.5875/1.290 = 18/x
Then I solve for x and subtract the x from the theta given. I'm not sure what i"m doing wrong!

The period calculation looks correct. Your last equation is not correct. Why are you not using the equation for theta from your original post?
theta= theta,o cos((2pi/T) t)