The square of Mark's age 3 years ago is 6 times the age he will be in 9 years. What is his age now?
15 years old
If we denote Mark's age today by
We know that
Thus by multiplying
Which, when simplified, leads us to a quadratic equation:
Hence the two possible answers are:
Obviously, you cannot be -3 years old, so he must be 15 years old.