What is the value of cosθ given that (-2, -3) is a point on the terminal side of θ?

Answer
-3/√13

Explanation
We are going to take angle ∅ to be at the origin (point (0,0)). 
The point at (-2, -3) means, 2 units to the left (negative side of x-axis) and 3 units downwards (negative side of y-xis). 
This will make a right triangle with a hypotenuse of √(2²+3²) = √13

The angle ∅ is in the 3rd quadrant where cosine is negative. 

cosine = adjacent/hypotenuse

The exact answer for cos ∅ = -2/√13