Since there are four tires, the total area is 0.018m^2 * 4 = 0.072m^2. Pressure = force / Area => 180*10^3 = Force/0.072 => Force= 12960 N. Then we plug the result for Force into Force =Mass* Acceleration => 12960 = mass*9.8m/s^2 => Mass = 1322.4kg. (Note that 9.8m/s^2 is the gravitational constant for acceleration.)
