Answer:
y = 3x,
Where, x = number of cans,
y = number of balls.
Explanation:
If the we consider the number of balls per can,
Number of balls ∝ number of can,
If x represents the number of cans and y represents the number of balls,
⇒ y ∝ x
⇒ y = kx
Where,
k = constant of proportionality,
if x = 3, y = 9

Thus, the required equation that shows the given relation would be,
y = 3x