csc theta = -8/5 :. sin theta = -5/8 ; pi < theta <(3pi)/2, theta is in third quadrant. Let P = perpendicular of rt triangle , H=hypotenuse of a rt. triangle and B= base of rt.triangle.
Sin theta = P/H :. P = -5 ; H=8 :. B= sqrt(H^2-P^2)= sqrt(64-25)=+-sqrt39 = -sqrt39 In 3 rd quadrant both P and B are negative.
:. tan theta = P/B = (-5)/-sqrt39 = 5/sqrt39 [Ans]