Vary Directly: y = k*x
Vary Inversely: y = k/x
Because x and y vary directly, our equation will look like the first one.
y = k*x
With the inputted values:
-16 = k * 4
Now, to find k, isolate the variable
k = -16/4 = -4
Your final equation will then look like this:
y = -4 * x
