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before this i found the position r of a particle moving in an xy plane is given by r = (1.00t3 - 8.00t) i + (7.00 - 4.00t4) j.

So I found r,v,a, for t=2.10s (all in m)

r=-7.54 i +-70.8 j

v=5.23 i + -148 j

a=12.6 i + -212 j

i first tried using v's i and j to make a triangle and find the angle from that by inverse tan (-148/5.23) and got -87degress or -212 but that was wrong...

any help?