Xiao Li is 6 years old, and her father is 36 years old. How many years later, her father's age will be three times that of Xiao Li?

Xiao Li is 6 years old, and her father is 36 years old. How many years later, her father's age will be three times that of Xiao Li?

Let's say that in another x years, Xiao Li's father is just twice as old as Xiao Li
3(6+X)=36+X
18+3X=36+X
-18=-2X
X=9