Answer:
35
Step-by-step explanation:
y is propotional to x, means, if x changes, y changes.
Considering, y depends on x only. Introduce a constant.
y = kx
When x = 9, y = 15,
=> 15 = k(9) => 5/3 = k
When x = 21, note that k is constant.
=> y = kx = (5/3)*21
= 35
