The table describes a proportional function. What is the constant of proportionality?

$\displaystyle{x}\usepackage{cmbright}\Huge\$
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1 6

2 12

3 18

4 24

$\displaystyle{x}\usepackage{cmbright}\Huge\$	$\displaystyle{y}\usepackage{cmbright}\Huge\$
1	6
2	12
3	18
4	24