The cosine of a right triangle is the adjacent over the hypotenuse. If the cosine is -3/5 then it must be a right triangle with sides 3, 4, & 5 since this is a Pythagorean triple. This means the sin will be -4/5. We know its negative because the tangent is positive so this is the 3rd quadrant where both Sine and Cosine are negative.
