Since this isn't part of the unit circle you're expected to memorize, use a calculator. Make sure you're in radians. arctan(-4/21) = -0.188 -0.188 is not in the range you want so add 2pi. -0.188 + 2pi = 6.095 In general, tanx = tan(x + pi). We want a number in [0, 2pi] so subtract pi for the second answer. 6.095 - pi = 2.953
