Hello there! The formula for constant of proportionality is k = y/x, which also is k = slope.
Equations with a constant of proportionality of 15 would be:
• k = 15/1
• k = 30/2
•k = 75/5
and so on. Anything that is a multiple of 15 and is placed as the numerator over the dividend used to receive a quotient of 15 would work. Hope this helps!