So I'm doing a Physics problem and have run into a snag with the online homework - every answer I've tried keeps getting rejected and I'm down to my last shot and I don't know what I'm doing wrong. The problem is as follows:
A 0.36 kg particle moves in an xy plane according to x(t) = -14 + 3t - 6$t^3$ and y(t) = 16+5t-8$t^2$, with x and y in meters and t in seconds. At t = 1.9 s, what are (a) the magnitude and (b) the angle (within (-180°, 180°] interval relative to the positive direction of the x axis) of the net force on the particle, and (c) what is the angle of the particle's direction of travel?
Now I've managed to figure out a and b, but the angle I get for c (-157.716 which I rounded to -157.7 degrees) seems to be incorrect, and I can't figure out why (I took the arctan of $\frac{-25.4}{-61.98}$ from calculating the x and y components from the derivative of x(t) and y(t))
Could someone explain to me what I missed?