A yard is equal in length to three feet. The function f(x) takes a measurement in yards (as input) and returns a measurement in feet (as output).

36.6
====================================
Work Shown:
f(x) = 3x
f(12.2) = 3*12.2
f(12.2) = 36.6
x is the length in yards, and f(x) is the length in feet.
12.2 yards = 36.6 feet
To convert from yards to feet, we multiply by 3. That explains how
as shown in the screenshot.