Let's denote Cindy's age which is unknown with X
We know that Cindy's age is 3 times her age 6 years ago.
6 years ago Cindy's age was the present age (X) - 6 : X-6
The fact that the present age is 3 times her age 6 year ago can be written as:
3*(X-6)
So, finally:
X=3*(X-6)
