Question 803739
You know that the age of the daughter plus the age of the father is 42 years.  You will use the equation x+y=42 (x=father y=daughter).  When its 7 years later and the father will be 3 times the age of his daughter. Both the father and daughter will have aged 7 years so you would add 14 to 42 and now your equation will be x+y=56.  you also need an equation to show that the father will be 3 times the age of his daughter.  You would use 3y=x. now to figure out the fathers age you would need to plug 3y=x into x+y=56.  3y+y=56 we will use this to find the daughters age first.  You will get y=14.  The father will be 3 times the age of his daughter.  You can now plug y=14 into 3y=x to find what the fathers age will be.  You now know the fathers age will be 42 in 7 years.  Since it is present day, simply subtract 7 from 42 to find his current age.  The father is 35 years old.