"Christian's father is three times as old as him"
Let a be christian's father's age and b be christian's age:
a=3b (equation 1)
"six years ago, his father's age was five times christian's age"
= a-6=5b-30 (equation 2)
substitute equation 1 to equation 2
substitute the value of b to equation 1
Christian's father is 36
Christian is 12