Hooke's law for three springs

In a lab, three springs were loaded and their stretches measured. You will make the program find each spring constant from the readings, pick the stiffest spring, and predict how far each one stretches under a new load.

The idea. Hooke's law, $F = kx$, makes a graph of $F$ against $x$ a straight line through the origin. Its slope is the spring constant $k$.

Your tasks.

1. Find the spring constant of each spring, from the slope of its readings.
2. Find the name of the stiffest spring.
3. Work out how far each spring stretches under a load of $7.5\ \text{N}$.