If Y Varies Directly as X, Find a Linear Function $$y = 18 \text { when } x = 21$$

Multiple Choice

If y varies directly as x, find a linear function which relates them.
- $$y = 18 \text { when } x = 21$$

A) $f ( x ) = 3 x$
B) $f ( x ) = \frac { 7 } { 6 } x$
C) $f ( x ) = \frac { 6 } { 7 } x$
D) $f ( x ) = x - 3$