A common factor of both 126 and 399 is 3 so we can divide both numbers by 3.
126 / 3 = 42
399 / 3 = 133
A common factor for both 42 and 133 is 7 so we can divide both numbers by 7.
42 / 7 = 6
133 / 7 = 19
19 is a prime number so we can't reduce it anymore, therefore 126/399 in its simplest form is 6/19