word problems

Hello everyone. I am stuck on a work question that we are required to solve using the matrix (or Gauss-Jordan) method. "A car rental company wants to buy 100 new cars. Compact cars cost $12,000 each, intermediate size cars cost $18,000 each, full size cars cost $24,000 each, and the company...

The Sum of three times a number and 18 is -39. Find the number. I was always confused with these problems and never understood them. Any help would be much appreciated!! Thank you!