When we say y is directly proportional to x, we write y = kx where k is some constant. Suppose we have a spring and we stretch it a distance of x beyond its natural length. Hookes law says that the force required to hold the spring x units beyond its natural length is directly proportional to x.

1. (a) A force of 40N is required to hold a spring that has been stretched from its natural length of 10cm to 15cm. Denote the force by y. Find an equation for y.
2. (b) Find the force needed to hold a spring from its natural length to 18cm.