let the first number is a
the second number is b
so, it is given One-third of a number decreased by the second number equals 11,

also, One-third of the first number increased by the second number is equal to 1.

add both the equation

so the first number is a = 18
put a = 18 in the first equation


so the second number is -6
so the numbers are 18 & -6