Given your ordered pair you would assume a = 5, b = 2
Set up both equations
5 + 2 = 7
7 = 7 (the numbers are equal so this correct)
2(5) - 8 = 2
10 - 8 = 2
2 = 2 (the numbers are equal so this is also correct)
Because both equations work with the ordered pair they are the solution of the given system.