so, using the data obtained from the three trials, you'll get three equations in a, b, c, which you can then solve by using Cramer's rule, Matrix method, or using a calculator.
2) Obviously the most accurate would be Linear Regression, (I think) but there is another method I can spot here.
For trials 1,2,3 and 4, the Pressure remains constant. Therefore, you can assume the the above equation as being only a function of volume.
So plot a graph of the 4 readings Vs
The slope of this graph gives you the value of b.
Now for trials 5,6,7 observe that the volume remains constant, hence plot the graph of
The slope of this graph would give you the value of c.
Now using c, and b obtained thus, you can estimate a.