Finding absolute zero from a gas thermometer

A constant-volume gas thermometer was read at six temperatures. Real readings scatter a little. You will make the program fit the best straight line through them, extend it to zero pressure to find absolute zero, and predict the reading at $300\ ^\circ\text{C}$.

The idea. At a steady volume, the pressure grows in a straight line with temperature: $p = at + b$. The line meets $p = 0$ at $t = -\dfrac{b}{a}$, absolute zero.

Your tasks.

1. Fit a straight line to the readings: find its slope a and intercept b.
2. Work out where the line meets zero pressure, t_zero.
3. Predict the pressure at $300\ ^\circ\text{C}$.