"mother's age will be three times greater than daughter's age"—Not sure what "three times greater than" means. I will interpret it as: "mother's age will be three times daughter's age."
In x years, mother will be 30+x and the daughter will be 6+x.
30+x = 3(6+x) = 18+3x
12 = 2x
x = 6
In 6 years, the mother will be 36 and the daughter will be 12.
