Answer:
If the point P(-3/5, y) lies on the unit circle, then we know that:
x^2 + y^2 = 1
Substituting x = -3/5, we get:
(-3/5)^2 + y^2 = 1
9/25 + y^2 = 1
y^2 = 1 - 9/25
y^2 = 16/25
Taking the square root of both sides, we get:
y = ±4/5
Since P is in the third quadrant (i.e., x is negative and y is negative), y must be negative. Therefore, y = -4/5.