Problem 504 is virtually the same as Problem 502: You are given a flow velocity and mass flow rate, and are asked to calculate the flow cross-sectional area. In Problem 502 the substance was steam and you had to use the property tables to obtain the density. Here in Problem 504, however, the fluid is air so you use the ideal gas equation of state to find the density.
Here is a problem that can be found as a solved example in the Thermodynamics textbook by Cengel and Boles. It is slightly more challenging than problem 504 and gives you additional insight/practice with the ideal gas equation.
Thermal and Fluid Systems Depth (PM) Exam Sample Problem #4
Air at 10°C and 80 kPa enters the diffuser of a jet engine steadily with a velocity of 200 m/s. The inlet area of the diffuser is 0.4 squared meters. The air leaves the diffuser with a velocity that is very small compared with the inlet velocity...