10 is the missing number.
Equation is y=kx. We have four chance to figure out k; hopefully they all give the same result
x=9, y=63, y=kx becomes 63=k(9) so k=63/9=7
x=4, y=28, 28=k(4), k=7
The constant of proportionality is 7; we check 2×7=14, 11×7=77
Answer: k=7, y=7x
