, homework trouble, springs and ocilatory motion!

plz help, homework trouble, springs and ocilatory motion!

1. The problem statement, all variables and given/known data
The position of a an air-track cart that is oscillating on a spring is given by (16.5cm)cos[(15.0s-1)t].
At what value t after t=0 is the cart first located at x=4.6 cm?


2. Relevant equations
x=Acos[(angular freqency)(t)+(phase constant)] i think is the only one but i dont get the question right, maybe its my math.


3. The attempt at a solution

@=phase constant, w= angular frequency

comparing x= 4.6cm=(16.5cm)cos[(15.0s-1)t] to:
X=Acos[wt+@]
inserting the information we get
0.046m=0.165m*cos1.5(t)
0.278=cos1.5t
t=0.278s

but incorrect.

 

the question itself is copied n pasted. the reason theres no phase constant is because apparently at that point phase constant = 0.. i dunno maybe im wrong about that but.. but the question is copied n pasted.

 

im confuzed right now :s

 

ugh it still didnt work out right, i got like 0.0289 >_<

 

then i got 0.289 when i tried again, but it still wasnt right

 

if i do that then i get 4.92 which apparently still aint right, unless im doing something wrong mathwise, i took the inverse cosine of 0.278 which was 73.9then divided it by 15. and got 4.92.

 

WOO who i got it! thanks!