SOLUTION: Tony's 17 years old and his father is 50 years old. How many years ago when his father was 4 times Tony's age?

Let x = How many years ago . . . 

At that time . . . 
   Tony was 17-x, and his father was 50-x years old.

The condition says 

4*(17 - x) = 50-x.

4*17 - 4x = 50 - x,

68 - 4x = 50 - x,

68 - 50 = 4x - x,

3x = 18,

x = 6.

Answer. It happened 6 years ago.

Check it yourself.