Under ideal conditions, the distance $d$ that a spring stretches varies directly as the force $f$ pulling on the string.

1. Express $d$ as a function of $f$ if a force of $10$ pounds stretches the spring $2$ inches.

   Hint (a):

   Don't forget the 'k' and to solve for it.

2. What force will stretch the spring $3$ inches?

   Answer (b):

   15 pounds