Nisha's father was 38 years of age when she was born while her mother was 36 years old when her brother four years younger to her was born. The difference of ages of her parents is:

Le t us assume the present Nisha's age be P years.
Then Nisha's younger brother Age will be P - 4.
Then father present age = 38 + P
Mother present Age = 36 + P - 4 = 32 + P
According to question,
Difference in age of parents = Father age - Mother age
Difference in age of parents = 38 + P - (32 + P)
Difference in age of parents = 38 + P - 32 - P = 6 years.