We know from the signs of x and y that this point is in QIII. Thus, the angle as measured from the positive x-axis is also in QIII.
First, use the Pyth. Thm. to determine the length of the ray from the origin to this point: (-3.84)^2 + (-2.87)^2 = L^2; L = sqrt (22.98), or approx. 4.79. The angle is 0.64 radian + pi radians, or 3.78 radians (which, as you can verify, is in QIII.
Thus, the point (-3.84, -2.87) becomes (4.79, 3.78 rad) in polar coordinates.
Let me know if you need help with the trigonometry involved here.