Answer:
In the given expression the constant of proportionality is k = 8.
Explanation:
The quantity P is said to be directly proportional to Q if they vary in proportion to each other.
Here, P ∝ Q ⇔ P = kQ : k = Proportionality Constant
Now, here given  y ∝ x and x = -2, y = -16
Now, y ∝ x ⇒ y = kx
or, (-16) = k (-2) 
or, k = 16/2 = 8
 ⇒  k = 8
 So, for k = 8, y ∝ x 
Hence, in the given expression the constant of proportionality is k = 8.