If sin of theta is 3/5, and theta in is quadrant 3, how do I find cos of theta?

Sin of theta AND cosine of theta in the THIRD quadrant are BOTH negative !

sin^2 + cos^2 = 1

(3/5)^2 + cos^2 = 1

cos^2 = 1 - (3/5)^2

cos^2 = 1- 9/25

cos^2 = 16/25

cos =  + - 4/5           

   BUT it is in quadrant 3  so it is NEGATIVE          cos = - 4/5     AND    sin = -3/5