Set up a system of linear equations to represent the scenario. Solve the system by using Gaussian elimination or Gauss-Jordan elimination. Robert stayed in three different cities (Washington, D. C., Atlanta, Georgia, and Dallas, Texas) for a total of 21 nights. He spent three times as many nights in Dallas as he did in Washington. The total cost for 21 nights (excluding tax) was $3330 . Determine the number of nights that he spent in each city.