Here you're discussing a linear relationship between x and y, also characterized as "direct proportion." Start with y = kx, where k is the "constant of variation."
Write y = kx, and then substitute the givens: 81 = k(27). Thus, k = 3.
Write y = 3x. We say "the constant of variation is 3."
Inverse variation is handled similarly; it's just that the relationship between x and y is no longer a linear one.