I understand the manipulation of 'pV=RT + Bp' and 'd=M/V' to get an equation in the form of 'y = mx + c', and after plotting the graph of p/d vs p, I multiply the gradient by M (molar mass) but still don't get the answer printed in my textbook.

Here are the given pressure and density values:

p /Torr 91.74 188.98 277.3 452.8 639.3 760

d /(g L-1) 0.225 0.456 0.664 1.062 1.468 1.734

I get: B = 2.119 L mol-1, B' = 1.140E-4 Torr-1 = 1.5003E-7 atm-1

The printed answer is: B = -4.4 L mol-1, B = -0.18 atm-1

Please could someone check if my answer is correct or if I have a problem with my method??