Final answer:
The equation representing the constant of proportionality (k) when two values are directly proportional is 'k = y/x'.
Step-by-step explanation:
When two values are directly proportional, it implies that as one value changes, the other changes by a consistent factor, known as the constant of proportionality. In mathematical terms, the relationship between two directly proportional values, let's say y and x, can be expressed as y = kx, where k represents the constant of proportionality. This means that for any value of x, you can find the corresponding value of y by multiplying x by k. Thus, if we want to solve for the constant k, we would rearrange the equation to k = y/x.